diff --git a/crates/voicevox_core_java_api/src/common.rs b/crates/voicevox_core_java_api/src/common.rs
index 2da808aae..2c6847f73 100644
--- a/crates/voicevox_core_java_api/src/common.rs
+++ b/crates/voicevox_core_java_api/src/common.rs
@@ -1,4 +1,4 @@
-use std::{error::Error as _, iter, str::Utf8Error};
+use std::{error::Error as _, iter};
 
 use derive_more::From;
 use jni::{objects::JThrowable, JNIEnv};
@@ -194,23 +194,16 @@ where
                     JavaApiError::Jni(error) => {
                         or_panic!(env.throw_new("java/lang/RuntimeException", error.to_string()))
                     }
-                    JavaApiError::Utf8(error) => {
+                    JavaApiError::Uuid(error) => {
                         or_panic!(
                             env.throw_new("java/lang/IllegalArgumentException", error.to_string())
                         )
                     }
-                    JavaApiError::Uuid(error) => {
+                    JavaApiError::DeJson(error) => {
                         or_panic!(
                             env.throw_new("java/lang/IllegalArgumentException", error.to_string())
                         )
                     }
-                    JavaApiError::Json(error) => {
-                        or_panic!(env.throw_new("java/lang/RuntimeException", error.to_string()))
-                    }
-                    JavaApiError::IllegalAccelerationMode => or_panic!(env.throw_new(
-                        "java/lang/IllegalArgumentException",
-                        "Invalid acceleration mode".to_owned(),
-                    )),
                 };
             }
             fallback
@@ -226,14 +219,8 @@ pub enum JavaApiError {
     #[from]
     Jni(jni::errors::Error),
 
-    #[from]
-    Utf8(Utf8Error),
-
     #[from]
     Uuid(uuid::Error),
 
-    #[from]
-    Json(serde_json::Error),
-
-    IllegalAccelerationMode,
+    DeJson(serde_json::Error),
 }
diff --git a/crates/voicevox_core_java_api/src/open_jtalk.rs b/crates/voicevox_core_java_api/src/open_jtalk.rs
index 98feaacb0..f26128d35 100644
--- a/crates/voicevox_core_java_api/src/open_jtalk.rs
+++ b/crates/voicevox_core_java_api/src/open_jtalk.rs
@@ -1,4 +1,7 @@
-use std::sync::{Arc, Mutex};
+use std::{
+    borrow::Cow,
+    sync::{Arc, Mutex},
+};
 
 use crate::common::throw_if_err;
 use jni::{
@@ -26,7 +29,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_OpenJtalk_rsNewWithInit
 ) {
     throw_if_err(env, (), |env| {
         let open_jtalk_dict_dir = env.get_string(&open_jtalk_dict_dir)?;
-        let open_jtalk_dict_dir = open_jtalk_dict_dir.to_str()?;
+        let open_jtalk_dict_dir = &*Cow::from(&open_jtalk_dict_dir);
 
         let internal = voicevox_core::OpenJtalk::new_with_initialize(open_jtalk_dict_dir)?;
         env.set_rust_field(&this, "handle", Arc::new(internal))?;
diff --git a/crates/voicevox_core_java_api/src/synthesizer.rs b/crates/voicevox_core_java_api/src/synthesizer.rs
index 94a5ed16f..3ddd1b47f 100644
--- a/crates/voicevox_core_java_api/src/synthesizer.rs
+++ b/crates/voicevox_core_java_api/src/synthesizer.rs
@@ -39,7 +39,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsNewWithIn
             } else if env.is_same_object(&acceleration_mode, gpu)? {
                 voicevox_core::AccelerationMode::Gpu
             } else {
-                return Err(JavaApiError::IllegalAccelerationMode);
+                panic!("予期しない`AccelerationMode`です: {acceleration_mode:?}");
             };
         }
         let cpu_num_threads = env.get_field(&builder, "cpuNumThreads", "I")?;
@@ -150,7 +150,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsAudioQuer
             )?
         };
 
-        let query_json = serde_json::to_string(&audio_query)?;
+        let query_json = serde_json::to_string(&audio_query).expect("should not fail");
 
         let j_audio_query = env.new_string(query_json)?;
 
@@ -178,7 +178,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsAudioQuer
             RUNTIME.block_on(internal.audio_query(&text, voicevox_core::StyleId::new(style_id)))?
         };
 
-        let query_json = serde_json::to_string(&audio_query)?;
+        let query_json = serde_json::to_string(&audio_query).expect("should not fail");
 
         let j_audio_query = env.new_string(query_json)?;
 
@@ -211,7 +211,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsAccentPhr
             )?
         };
 
-        let query_json = serde_json::to_string(&accent_phrases)?;
+        let query_json = serde_json::to_string(&accent_phrases).expect("should not fail");
 
         let j_accent_phrases = env.new_string(query_json)?;
 
@@ -241,7 +241,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsAccentPhr
             )?
         };
 
-        let query_json = serde_json::to_string(&accent_phrases)?;
+        let query_json = serde_json::to_string(&accent_phrases).expect("should not fail");
 
         let j_accent_phrases = env.new_string(query_json)?;
 
@@ -259,7 +259,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplaceMo
     throw_if_err(env, std::ptr::null_mut(), |env| {
         let accent_phrases_json: String = env.get_string(&accent_phrases_json)?.into();
         let accent_phrases: Vec<voicevox_core::AccentPhraseModel> =
-            serde_json::from_str(&accent_phrases_json)?;
+            serde_json::from_str(&accent_phrases_json).map_err(JavaApiError::DeJson)?;
         let style_id = style_id as u32;
 
         let internal = env
@@ -273,7 +273,8 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplaceMo
             )?
         };
 
-        let replaced_accent_phrases_json = serde_json::to_string(&replaced_accent_phrases)?;
+        let replaced_accent_phrases_json =
+            serde_json::to_string(&replaced_accent_phrases).expect("should not fail");
 
         Ok(env.new_string(replaced_accent_phrases_json)?.into_raw())
     })
@@ -291,7 +292,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplacePh
     throw_if_err(env, std::ptr::null_mut(), |env| {
         let accent_phrases_json: String = env.get_string(&accent_phrases_json)?.into();
         let accent_phrases: Vec<voicevox_core::AccentPhraseModel> =
-            serde_json::from_str(&accent_phrases_json)?;
+            serde_json::from_str(&accent_phrases_json).map_err(JavaApiError::DeJson)?;
         let style_id = style_id as u32;
 
         let internal = env
@@ -306,7 +307,8 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplacePh
             )?
         };
 
-        let replaced_accent_phrases_json = serde_json::to_string(&replaced_accent_phrases)?;
+        let replaced_accent_phrases_json =
+            serde_json::to_string(&replaced_accent_phrases).expect("should not fail");
 
         Ok(env.new_string(replaced_accent_phrases_json)?.into_raw())
     })
@@ -322,7 +324,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplaceMo
     throw_if_err(env, std::ptr::null_mut(), |env| {
         let accent_phrases_json: String = env.get_string(&accent_phrases_json)?.into();
         let accent_phrases: Vec<voicevox_core::AccentPhraseModel> =
-            serde_json::from_str(&accent_phrases_json)?;
+            serde_json::from_str(&accent_phrases_json).map_err(JavaApiError::DeJson)?;
         let style_id = style_id as u32;
 
         let internal = env
@@ -336,7 +338,8 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsReplaceMo
             )?
         };
 
-        let replaced_accent_phrases_json = serde_json::to_string(&replaced_accent_phrases)?;
+        let replaced_accent_phrases_json =
+            serde_json::to_string(&replaced_accent_phrases).expect("should not fail");
 
         Ok(env.new_string(replaced_accent_phrases_json)?.into_raw())
     })
@@ -352,7 +355,8 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_Synthesizer_rsSynthesis
 ) -> jobject {
     throw_if_err(env, std::ptr::null_mut(), |env| {
         let audio_query: String = env.get_string(&query_json)?.into();
-        let audio_query: voicevox_core::AudioQueryModel = serde_json::from_str(&audio_query)?;
+        let audio_query: voicevox_core::AudioQueryModel =
+            serde_json::from_str(&audio_query).map_err(JavaApiError::DeJson)?;
         let style_id = style_id as u32;
 
         let internal = env
diff --git a/crates/voicevox_core_java_api/src/user_dict.rs b/crates/voicevox_core_java_api/src/user_dict.rs
index 4f7bc78d8..e85085a34 100644
--- a/crates/voicevox_core_java_api/src/user_dict.rs
+++ b/crates/voicevox_core_java_api/src/user_dict.rs
@@ -1,7 +1,10 @@
 use jni::objects::JClass;
-use std::sync::{Arc, Mutex};
+use std::{
+    borrow::Cow,
+    sync::{Arc, Mutex},
+};
 
-use crate::common::throw_if_err;
+use crate::common::{throw_if_err, JavaApiError};
 use jni::{
     objects::{JObject, JString},
     sys::jobject,
@@ -34,9 +37,10 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsAddWord<'loc
             .clone();
 
         let word_json = env.get_string(&word_json)?;
-        let word_json = word_json.to_str()?;
+        let word_json = &Cow::from(&word_json);
 
-        let word: voicevox_core::UserDictWord = serde_json::from_str(word_json)?;
+        let word: voicevox_core::UserDictWord =
+            serde_json::from_str(word_json).map_err(JavaApiError::DeJson)?;
 
         let uuid = {
             let mut internal = internal.lock().unwrap();
@@ -63,11 +67,12 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsUpdateWord<'
             .clone();
 
         let uuid = env.get_string(&uuid)?;
-        let uuid = uuid.to_str()?.parse()?;
+        let uuid = Cow::from(&uuid).parse()?;
         let word_json = env.get_string(&word_json)?;
-        let word_json = word_json.to_str()?;
+        let word_json = &Cow::from(&word_json);
 
-        let word: voicevox_core::UserDictWord = serde_json::from_str(word_json)?;
+        let word: voicevox_core::UserDictWord =
+            serde_json::from_str(word_json).map_err(JavaApiError::DeJson)?;
 
         {
             let mut internal = internal.lock().unwrap();
@@ -90,7 +95,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsRemoveWord<'
             .clone();
 
         let uuid = env.get_string(&uuid)?;
-        let uuid = uuid.to_str()?.parse()?;
+        let uuid = Cow::from(&uuid).parse()?;
 
         {
             let mut internal = internal.lock().unwrap();
@@ -137,7 +142,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsLoad<'local>
             .clone();
 
         let path = env.get_string(&path)?;
-        let path = path.to_str()?;
+        let path = &Cow::from(&path);
 
         {
             let mut internal = internal.lock().unwrap();
@@ -160,7 +165,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsSave<'local>
             .clone();
 
         let path = env.get_string(&path)?;
-        let path = path.to_str()?;
+        let path = &Cow::from(&path);
 
         {
             let internal = internal.lock().unwrap();
@@ -183,7 +188,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsGetWords<'lo
 
         let words = {
             let internal = internal.lock().unwrap();
-            serde_json::to_string(internal.words())?
+            serde_json::to_string(internal.words()).expect("should not fail")
         };
 
         let words = env.new_string(words)?;
@@ -211,7 +216,7 @@ extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsToZenkaku<'local>(
 ) -> jobject {
     throw_if_err(env, std::ptr::null_mut(), |env| {
         let text = env.get_string(&text)?;
-        let text = text.to_str()?;
+        let text = &Cow::from(&text);
 
         let text = voicevox_core::__internal::to_zenkaku(text);
 
@@ -228,7 +233,7 @@ extern "system" fn Java_jp_hiroshiba_voicevoxcore_UserDict_rsValidatePronunciati
 ) {
     throw_if_err(env, (), |env| {
         let text = env.get_string(&text)?;
-        let text = text.to_str()?;
+        let text = &Cow::from(&text);
 
         voicevox_core::__internal::validate_pronunciation(text)?;
 
diff --git a/crates/voicevox_core_java_api/src/voice_model.rs b/crates/voicevox_core_java_api/src/voice_model.rs
index d94904877..cd971a1f7 100644
--- a/crates/voicevox_core_java_api/src/voice_model.rs
+++ b/crates/voicevox_core_java_api/src/voice_model.rs
@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use std::{borrow::Cow, sync::Arc};
 
 use crate::common::{throw_if_err, RUNTIME};
 use jni::{
@@ -15,7 +15,7 @@ unsafe extern "system" fn Java_jp_hiroshiba_voicevoxcore_VoiceModel_rsFromPath<'
 ) {
     throw_if_err(env, (), |env| {
         let model_path = env.get_string(&model_path)?;
-        let model_path = model_path.to_str()?;
+        let model_path = &*Cow::from(&model_path);
 
         let internal = RUNTIME.block_on(voicevox_core::VoiceModel::from_path(model_path))?;