diff --git a/voicevox_engine/engine_manifest.py b/voicevox_engine/engine_manifest.py
index 0ea321be3..9307416e5 100644
--- a/voicevox_engine/engine_manifest.py
+++ b/voicevox_engine/engine_manifest.py
@@ -7,14 +7,16 @@
 
 import json
 from base64 import b64encode
+from dataclasses import asdict, dataclass
 from pathlib import Path
 from typing import TypeAlias
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, TypeAdapter
 from pydantic.json_schema import SkipJsonSchema
 
 
-class FeatureSupportJson(BaseModel):
+@dataclass(frozen=True)
+class FeatureSupportJson:
     """`engine_manifest.json` の機能サポート状況"""
 
     type: str
@@ -22,7 +24,8 @@ class FeatureSupportJson(BaseModel):
     name: str
 
 
-class SupportedFeaturesJson(BaseModel):
+@dataclass(frozen=True)
+class SupportedFeaturesJson:
     """`engine_manifest.json` のサポート機能一覧"""
 
     adjust_mora_pitch: FeatureSupportJson
@@ -37,7 +40,8 @@ class SupportedFeaturesJson(BaseModel):
     manage_library: FeatureSupportJson
 
 
-class EngineManifestJson(BaseModel):
+@dataclass(frozen=True)
+class EngineManifestJson:
     """`engine_manifest.json` のコンテンツ"""
 
     manifest_version: str
@@ -57,6 +61,9 @@ class EngineManifestJson(BaseModel):
     supported_features: SupportedFeaturesJson
 
 
+_manifest_json_adapter = TypeAdapter(EngineManifestJson)
+
+
 class UpdateInfo(BaseModel):
     """
     エンジンのアップデート情報
@@ -135,7 +142,7 @@ def load_manifest(manifest_path: Path) -> EngineManifest:
     """エンジンマニフェストを指定ファイルから読み込む。"""
 
     root_dir = manifest_path.parent
-    manifest = EngineManifestJson.model_validate_json(manifest_path.read_bytes())
+    manifest = _manifest_json_adapter.validate_json(manifest_path.read_bytes())
     return EngineManifest(
         manifest_version=manifest.manifest_version,
         name=manifest.name,
@@ -161,6 +168,6 @@ def load_manifest(manifest_path: Path) -> EngineManifest:
         ],
         supported_features={
             key: item["value"]
-            for key, item in manifest.supported_features.model_dump().items()
+            for key, item in asdict(manifest.supported_features).items()
         },
     )