diff --git a/crates/voicevox_core/src/voice_model.rs b/crates/voicevox_core/src/voice_model.rs
index aa17d798a..af1953aa1 100644
--- a/crates/voicevox_core/src/voice_model.rs
+++ b/crates/voicevox_core/src/voice_model.rs
@@ -12,7 +12,7 @@ pub struct VoiceModel {}
 
 impl VoiceModel {
     /// 与えられたパスからモデルを取得する
-    pub fn from_path(path: impl AsRef<Path>) -> Result<Self> {}
+    pub async fn from_path(path: impl AsRef<Path>) -> Result<Self> {}
     /// ランダムに発行されたこのモデルのIdを返す
     pub fn id(&self) -> &VoiceModelId {}
     /// このモデルのメタ情報を返す
diff --git a/crates/voicevox_core/src/voice_synthesizer.rs b/crates/voicevox_core/src/voice_synthesizer.rs
index 068c5abd2..89c4653d6 100644
--- a/crates/voicevox_core/src/voice_synthesizer.rs
+++ b/crates/voicevox_core/src/voice_synthesizer.rs
@@ -4,13 +4,13 @@ use super::*;
 pub struct VoiceSynthesizer {}
 impl VoiceSynthesizer {
     /// コンストラクタ兼初期化
-    pub fn new_with_initialize(options: InitializeOptions) -> Self {}
+    pub async fn new_with_initialize(options: InitializeOptions) -> Self {}
 
     /// 音声合成モデルを読み込む
-    pub fn load_model(&mut self, model: VoiceModel) -> Result<()> {}
+    pub async fn load_model(&mut self, model: VoiceModel) -> Result<()> {}
 
     /// 指定したモデルIdの音声合成モデルを開放する
-    pub fn unload_model(&mut self, model_id: VoiceModelId) -> Result<()> {}
+    pub async fn unload_model(&mut self, model_id: VoiceModelId) -> Result<()> {}
 
     /// 指定したモデルIdの音声合成モデルが読み込まれているか判定する
     pub fn is_loaded_model(&self, model_id: VoiceModelId) -> bool {}
@@ -19,5 +19,5 @@ impl VoiceSynthesizer {
     pub fn metas(&self) -> &VoiceModelMeta {}
 
     /// 音声合成を行う
-    pub fn synthesis(&self, style_id: &StyleId) -> Result<Vec<u8>> {}
+    pub async fn synthesis(&self, style_id: &StyleId) -> Result<Vec<u8>> {}
 }