diff --git a/mixnet/mixnet.py b/mixnet/mixnet.py
index fbc1ece9..caa75f47 100644
--- a/mixnet/mixnet.py
+++ b/mixnet/mixnet.py
@@ -2,20 +2,10 @@
 
 import random
 from dataclasses import dataclass
-from typing import List, TypeAlias
+from typing import List
 
-from cryptography.hazmat.primitives.asymmetric.x25519 import (
-    X25519PrivateKey,
-    X25519PublicKey,
-)
-from pysphinx.node import Node
-
-from mixnet.bls import BlsPrivateKey, BlsPublicKey
 from mixnet.fisheryates import FisherYates
-
-NodeId: TypeAlias = BlsPublicKey
-# 32-byte that represents an IP address and a port of a mix node.
-NodeAddress: TypeAlias = bytes
+from mixnet.node import MixNode
 
 
 @dataclass
@@ -49,22 +39,6 @@ def choose_mixnode(self) -> MixNode:
         return random.choice(self.mix_nodes)
 
 
-@dataclass
-class MixNode:
-    identity_private_key: BlsPrivateKey
-    encryption_private_key: X25519PrivateKey
-    addr: NodeAddress
-
-    def identity_public_key(self) -> BlsPublicKey:
-        return self.identity_private_key.get_g1()
-
-    def encryption_public_key(self) -> X25519PublicKey:
-        return self.encryption_private_key.public_key()
-
-    def sphinx_node(self) -> Node:
-        return Node(self.encryption_private_key, self.addr)
-
-
 @dataclass
 class MixnetTopology:
     layers: List[List[MixNode]]
diff --git a/mixnet/node.py b/mixnet/node.py
new file mode 100644
index 00000000..defd5b36
--- /dev/null
+++ b/mixnet/node.py
@@ -0,0 +1,174 @@
+from __future__ import annotations
+
+import queue
+import threading
+import time
+from dataclasses import dataclass
+from threading import Thread
+from typing import Tuple, TypeAlias
+
+from cryptography.hazmat.primitives.asymmetric.x25519 import (
+    X25519PrivateKey,
+    X25519PublicKey,
+)
+from pysphinx.node import Node
+from pysphinx.sphinx import (
+    Payload,
+    ProcessedFinalHopPacket,
+    ProcessedForwardHopPacket,
+    SphinxPacket,
+    UnknownHeaderTypeError,
+)
+
+from mixnet.bls import BlsPrivateKey, BlsPublicKey
+from mixnet.poisson import poisson_interval_sec
+
+NodeId: TypeAlias = BlsPublicKey
+# 32-byte that represents an IP address and a port of a mix node.
+NodeAddress: TypeAlias = bytes
+
+PacketQueue: TypeAlias = "queue.Queue[Tuple[NodeAddress, SphinxPacket]]"
+PacketPayloadQueue: TypeAlias = (
+    "queue.Queue[Tuple[NodeAddress, SphinxPacket | Payload]]"
+)
+
+
+@dataclass
+class MixNode:
+    identity_private_key: BlsPrivateKey
+    encryption_private_key: X25519PrivateKey
+    addr: NodeAddress
+
+    def identity_public_key(self) -> BlsPublicKey:
+        return self.identity_private_key.get_g1()
+
+    def encryption_public_key(self) -> X25519PublicKey:
+        return self.encryption_private_key.public_key()
+
+    def sphinx_node(self) -> Node:
+        return Node(self.encryption_private_key, self.addr)
+
+    def start(
+        self,
+        delay_rate_per_min: int,
+        inbound_socket: PacketQueue,
+        outbound_socket: PacketPayloadQueue,
+    ) -> MixNodeRunner:
+        thread = MixNodeRunner(
+            self.encryption_private_key,
+            delay_rate_per_min,
+            inbound_socket,
+            outbound_socket,
+        )
+        thread.daemon = True
+        thread.start()
+        return thread
+
+
+class MixNodeRunner(Thread):
+    """
+    Read SphinxPackets from inbound socket and spawn a thread for each packet to process it.
+
+    This thread approximates a M/M/inf queue.
+    """
+
+    def __init__(
+        self,
+        encryption_private_key: X25519PrivateKey,
+        delay_rate_per_min: int,  # Poisson rate parameter: mu
+        inbound_socket: PacketQueue,
+        outbound_socket: PacketPayloadQueue,
+    ):
+        super().__init__()
+        self.encryption_private_key = encryption_private_key
+        self.delay_rate_per_min = delay_rate_per_min
+        self.inbound_socket = inbound_socket
+        self.outbound_socket = outbound_socket
+        self.num_processing = AtomicInt(0)
+
+    def run(self) -> None:
+        # Here in Python, this thread is implemented in synchronous manner.
+        # In the real implementation, consider implementing this in asynchronous if possible,
+        # to approximate a M/M/inf queue
+        while True:
+            _, packet = self.inbound_socket.get()
+            thread = MixNodePacketProcessor(
+                packet,
+                self.encryption_private_key,
+                self.delay_rate_per_min,
+                self.outbound_socket,
+                self.num_processing,
+            )
+            thread.daemon = True
+            self.num_processing.add(1)
+            thread.start()
+
+    def num_jobs(self) -> int:
+        """
+        Return the number of packets that are being processed or still in the inbound socket.
+
+        If this thread works as a M/M/inf queue completely,
+        the number of packets that are still in the inbound socket must be always 0.
+        """
+        return self.num_processing.get() + self.inbound_socket.qsize()
+
+
+class MixNodePacketProcessor(Thread):
+    """
+    Process a single packet with a delay that follows exponential distribution,
+    and forward it to the next mix node or the mix destination
+
+    This thread is a single server (worker) in a M/M/inf queue that MixNodeRunner approximates.
+    """
+
+    def __init__(
+        self,
+        packet: SphinxPacket,
+        encryption_private_key: X25519PrivateKey,
+        delay_rate_per_min: int,  # Poisson rate parameter: mu
+        outbound_socket: PacketPayloadQueue,
+        num_processing: AtomicInt,
+    ):
+        super().__init__()
+        self.packet = packet
+        self.encryption_private_key = encryption_private_key
+        self.delay_rate_per_min = delay_rate_per_min
+        self.outbound_socket = outbound_socket
+        self.num_processing = num_processing
+
+    def run(self) -> None:
+        delay_sec = poisson_interval_sec(self.delay_rate_per_min)
+        time.sleep(delay_sec)
+
+        processed = self.packet.process(self.encryption_private_key)
+        match processed:
+            case ProcessedForwardHopPacket():
+                self.outbound_socket.put(
+                    (processed.next_node_address, processed.next_packet)
+                )
+            case ProcessedFinalHopPacket():
+                self.outbound_socket.put(
+                    (processed.destination_node_address, processed.payload)
+                )
+            case _:
+                raise UnknownHeaderTypeError
+
+        self.num_processing.sub(1)
+
+
+class AtomicInt:
+    def __init__(self, initial: int) -> None:
+        self.lock = threading.Lock()
+        self.value = initial
+
+    def add(self, v: int):
+        with self.lock:
+            self.value += v
+
+    def sub(self, v: int):
+        with self.lock:
+            self.value -= v
+
+    def get(self) -> int:
+        with self.lock:
+            return self.value
diff --git a/mixnet/poisson.py b/mixnet/poisson.py
new file mode 100644
index 00000000..86a4b66a
--- /dev/null
+++ b/mixnet/poisson.py
@@ -0,0 +1,13 @@
+import numpy
+
+
+def poisson_interval_sec(rate_per_min: int) -> float:
+    # If events occur in a Poisson distribution with rate_per_min,
+    # the interval between events follows the exponential distribution
+    # with the rate_per_min (i.e. with the scale 1/rate_per_min).
+    interval_min = numpy.random.exponential(scale=1 / rate_per_min, size=1)[0]
+    return interval_min * 60
+
+
+def poisson_mean_interval_sec(rate_per_min: int) -> float:
+    return 1 / rate_per_min * 60
diff --git a/mixnet/test_node.py b/mixnet/test_node.py
new file mode 100644
index 00000000..8783e9d7
--- /dev/null
+++ b/mixnet/test_node.py
@@ -0,0 +1,114 @@
+import queue
+import threading
+import time
+from datetime import datetime
+from typing import Tuple
+from unittest import TestCase
+
+import numpy
+import timeout_decorator
+from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
+from pysphinx.sphinx import SphinxPacket
+
+from mixnet.bls import generate_bls
+from mixnet.mixnet import Mixnet, MixnetTopology
+from mixnet.node import MixNode, NodeAddress, PacketPayloadQueue, PacketQueue
+from mixnet.packet import PacketBuilder
+from mixnet.poisson import poisson_interval_sec, poisson_mean_interval_sec
+from mixnet.utils import random_bytes
+
+
+class TestMixNodeRunner(TestCase):
+    @timeout_decorator.timeout(180)
+    def test_mixnode_runner_emission_rate(self):
+        """
+        Test if MixNodeRunner works as a M/M/inf queue.
+
+        If inputs are arrived at Poisson rate `lambda`,
+        and if processing is delayed according to an exponential distribution with a rate `mu`,
+        the rate of outputs should be `lambda`.
+        """
+        mixnet, topology = self.init()
+        inbound_socket: PacketQueue = queue.Queue()
+        outbound_socket: PacketPayloadQueue = queue.Queue()
+
+        packet, route = PacketBuilder.real(b"msg", mixnet, topology).next()
+
+        delay_rate_per_min = 30  # mu (= 2s delay on average)
+        # Start only the first mix node for testing
+        runner = route[0].start(delay_rate_per_min, inbound_socket, outbound_socket)
+
+        # Send packets to the first mix node in a Poisson distribution
+        packet_count = 100
+        emission_rate_per_min = 120  # lambda (= 2msg/sec)
+        sender = threading.Thread(
+            target=self.send_packets,
+            args=(
+                inbound_socket,
+                packet,
+                route[0].addr,
+                packet_count,
+                emission_rate_per_min,
+            ),
+        )
+        sender.daemon = True
+        sender.start()
+
+        # Calculate intervals between outputs and gather num_jobs in the first mix node.
+        intervals = []
+        num_jobs = []
+        ts = datetime.now()
+        for _ in range(packet_count):
+            _ = outbound_socket.get()
+            now = datetime.now()
+            intervals.append((now - ts).total_seconds())
+            num_jobs.append(runner.num_jobs())
+            ts = now
+        # Remove the first interval that would be much larger than other intervals,
+        # because of the delay in mix node.
+        intervals = intervals[1:]
+        num_jobs = num_jobs[1:]
+
+        # Check if the emission rate of the first mix node is the same as
+        # the emission rate of the message sender, but with a delay.
+        # If outputs follow the Poisson distribution with a rate `lambda`,
+        # a mean interval between outputs must be `1/lambda`.
+        self.assertAlmostEqual(
+            float(numpy.mean(intervals)),
+            poisson_mean_interval_sec(emission_rate_per_min),
+            delta=1.0,
+        )
+        # If runner is a M/M/inf queue,
+        # a mean number of jobs being processed/scheduled in the runner must be `lambda/mu`.
+        self.assertAlmostEqual(
+            float(numpy.mean(num_jobs)),
+            round(emission_rate_per_min / delay_rate_per_min),
+            delta=1.0,
+        )
+
+    @staticmethod
+    def send_packets(
+        inbound_socket: PacketQueue,
+        packet: SphinxPacket,
+        node_addr: NodeAddress,
+        cnt: int,
+        rate_per_min: int,
+    ):
+        for _ in range(cnt):
+            time.sleep(poisson_interval_sec(rate_per_min))
+            inbound_socket.put((node_addr, packet))
+
+    @staticmethod
+    def init() -> Tuple[Mixnet, MixnetTopology]:
+        mixnet = Mixnet(
+            [
+                MixNode(
+                    generate_bls(),
+                    X25519PrivateKey.generate(),
+                    random_bytes(32),
+                )
+                for _ in range(12)
+            ]
+        )
+        topology = mixnet.build_topology(b"entropy", 3, 3)
+        return mixnet, topology
diff --git a/mixnet/test_packet.py b/mixnet/test_packet.py
index 676da7e1..e306e3a9 100644
--- a/mixnet/test_packet.py
+++ b/mixnet/test_packet.py
@@ -5,7 +5,8 @@
 from pysphinx.sphinx import ProcessedFinalHopPacket, SphinxPacket
 
 from mixnet.bls import generate_bls
-from mixnet.mixnet import Mixnet, MixnetTopology, MixNode
+from mixnet.mixnet import Mixnet, MixnetTopology
+from mixnet.node import MixNode
 from mixnet.packet import (
     Fragment,
     MessageFlag,
diff --git a/requirements.txt b/requirements.txt
index 0013983a..81d737e2 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,4 +6,5 @@ pycparser==2.21
 pysphinx==0.0.1
 scipy==1.11.4
 setuptools==69.0.3
+timeout-decorator==0.5.0
 wheel==0.42.0