diff --git a/firmware/SensorLogger.cpp b/firmware/SensorLogger.cpp
index 9f388ce..8695591 100644
--- a/firmware/SensorLogger.cpp
+++ b/firmware/SensorLogger.cpp
@@ -58,6 +58,7 @@ public:
   void onConnect(BLEServer*) override {
     // 接続中は、画面に接続中であることを表示できます。
     logger_->bluetoothConnected_ = true;
+    Serial.println("[INFO] BLE client connected");
   }
 
   void onDisconnect(BLEServer* server) override {
@@ -66,6 +67,7 @@ public:
     logger_->commandBuffer_ = "";
     logger_->lastAdvertise_ = millis();
     server->startAdvertising();
+    Serial.println("[WARNING] BLE client disconnected; advertising restarted");
   }
 
 private:
@@ -82,9 +84,19 @@ void SensorLogger::begin(uint8_t soilPin) {
   // 未接続時にライブラリへアクセスすると、不要なI2Cエラーや待ち時間が発生します。
   if (isI2cDevicePresent(0x38)) {
     aht20Ready = aht20.begin();
+    if (!aht20Ready) {
+      Serial.println("[ERROR] AHT20 initialization failed");
+    }
+  } else {
+    Serial.println("[WARNING] AHT20 not detected at 0x38");
   }
   if (isI2cDevicePresent(0x23)) {
     bh1750Ready = bh1750.begin(BH1750::CONTINUOUS_HIGH_RES_MODE);
+    if (!bh1750Ready) {
+      Serial.println("[ERROR] BH1750 initialization failed");
+    }
+  } else {
+    Serial.println("[WARNING] BH1750 not detected at 0x23");
   }
 
   // ESP32固有の番号を名前に加え、同じ名前の機器と区別します。
@@ -93,6 +105,7 @@ void SensorLogger::begin(uint8_t soilPin) {
   snprintf(name, sizeof(name), "%s-%05llX", kBleDeviceNamePrefix, mac & 0xFFFFF);
   bluetoothName_ = name;
   BLEDevice::init(bluetoothName_.c_str());
+  Serial.printf("[INFO] BLE initialized as '%s'\n", bluetoothName_.c_str());
 
   // BLEサーバーと、センサー通信用のサービスを作ります。
   server_ = BLEDevice::createServer();
@@ -119,6 +132,7 @@ void SensorLogger::begin(uint8_t soilPin) {
   advertising->setMinPreferred(0x06);
   advertising->setMinPreferred(0x12);
   BLEDevice::startAdvertising();
+  Serial.println("[INFO] BLE advertising started");
 
   // 起動直後にも1回計測して、画面に値を表示できるようにします。
   readAndStore();
@@ -281,6 +295,8 @@ void SensorLogger::receiveBluetoothData(const String& data) {
       commandBuffer_ += character;
     } else if (commandBuffer_.length() < 64) {
       commandBuffer_ += character;
+    } else if (commandBuffer_.length() == 64) {
+      Serial.println("[ERROR] BLE command is too long and was truncated");
     }
   }
 }
@@ -340,9 +356,12 @@ void SensorLogger::processCommand(const String& command) {
       txCharacteristic_->setValue("OK_TIME\n");
       txCharacteristic_->notify();
     } else {
+      Serial.printf("[ERROR] Invalid TIME command: %s\n", command.c_str());
       txCharacteristic_->setValue("ERROR_TIME\n");
       txCharacteristic_->notify();
     }
+  } else {
+    Serial.printf("[WARNING] Unknown BLE command: %s\n", command.c_str());
   }
 }
 
diff --git a/firmware/firmware.ino b/firmware/firmware.ino
index 0efc5a8..bfac6f6 100644
--- a/firmware/firmware.ino
+++ b/firmware/firmware.ino
@@ -183,6 +183,8 @@ void avrc_metadata_callback(uint8_t id, const uint8_t *text) {
   } else if (id == ESP_AVRC_MD_ATTR_ARTIST) {
     currentArtist = (char*)text;
     updated = true;
+  } else {
+    Serial.printf("[WARNING] Unsupported AVRCP metadata id: %u\n", id);
   }
 
   if (updated && bluetoothConnected && musicPlaying) {
@@ -201,6 +203,8 @@ void volume_changed_callback(int volume) {
 void audio_state_changed_callback(esp_a2d_audio_state_t state, void *) {
   // 音声が流れ始めたかどうかを保存し、LEDと画面を更新します。
   musicPlaying = state == ESP_A2D_AUDIO_STATE_STARTED;
+  Serial.printf("[INFO] Audio state changed: %d (playing=%s)\n",
+    state, musicPlaying ? "yes" : "no");
   neoPixelController.setPlaying(musicPlaying);
   updateDisplay();
 }
@@ -208,6 +212,8 @@ void audio_state_changed_callback(esp_a2d_audio_state_t state, void *) {
 void connection_state_changed_callback(esp_a2d_connection_state_t state, void *) {
   // スマートフォンとの接続状態を保存します。
   bluetoothConnected = state == ESP_A2D_CONNECTION_STATE_CONNECTED;
+  Serial.printf("[INFO] A2DP connection state changed: %d (connected=%s)\n",
+    state, bluetoothConnected ? "yes" : "no");
   if (!bluetoothConnected) {
     musicPlaying = false;
   }
@@ -217,6 +223,7 @@ void connection_state_changed_callback(esp_a2d_connection_state_t state, void *)
 void setup() {
   // setupは、電源を入れた直後に1回だけ実行されます。
   Serial.begin(115200);
+  Serial.println("[INFO] Firmware starting");
 
   neoPixelController.begin(18);
 
@@ -237,6 +244,9 @@ void setup() {
     u8g2.begin();
     u8g2.enableUTF8Print(); // UTF-8（日本語）描画を有効化
     oledReady = true;
+    Serial.printf("[INFO] OLED detected at 0x%02X\n", oledAddress);
+  } else {
+    Serial.println("[WARNING] OLED not detected at 0x3C or 0x3D");
   }
 
   sensorLogger.begin(32);
@@ -262,6 +272,7 @@ void setup() {
 
   // Bluetoothスピーカーとして起動します。
   a2dp_sink.start(kA2dpDeviceName);
+  Serial.printf("[INFO] A2DP started as '%s'\n", kA2dpDeviceName);
 }
 
 void loop() {