A portable electronic key device used with a communication peer device and capable of performing bidirectional communication with the communication peer device, wherein the bidirectional communication is initiated by a bidirectional communication initiation signal transmitted from the communication peer device, the portable electronic key device being characterized by : a vibration sensor configured to detect vibration of the portable electronic key device ; a reception antenna configured to receive the bidirectional communication initiation signal; a transmission antenna configured to transmit a response signal to the communication peer device in response to the bidirectional communication initiation signal; and a controller connected to the vibration sensor , wherein the controller is configured to: check a status of the vibration sensor to determine whether or not the vibration sensor is defective; activate a bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller receives a vibration detection signal from the vibration sensor ; deactivate the bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller does not receive the vibration detection signal from the vibration sensor ; and activate the bidirectional communication initiation signal waiting function when the vibration sensor is defective regardless of whether or not the vibration detection signal is received from the vibration sensor . a vibration sensor configured to detect vibration of the portable electronic key device ; a reception antenna configured to receive the bidirectional communication initiation signal; a transmission antenna configured to transmit a response signal to the communication peer device in response to the bidirectional communication initiation signal; and a controller connected to the vibration sensor , wherein the controller is configured to: check a status of the vibration sensor to determine whether or not the vibration sensor is defective; activate a bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller receives a vibration detection signal from the vibration sensor ; deactivate the bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller does not receive the vibration detection signal from the vibration sensor ; and activate the bidirectional communication initiation signal waiting function when the vibration sensor is defective regardless of whether or not the vibration detection signal is received from the vibration sensor . check a status of the vibration sensor to determine whether or not the vibration sensor is defective; activate a bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller receives a vibration detection signal from the vibration sensor ; deactivate the bidirectional communication initiation signal waiting function when the vibration sensor is functioning normally and the controller does not receive the vibration detection signal from the vibration sensor ; and activate the bidirectional communication initiation signal waiting function when the vibration sensor is defective regardless of whether or not the vibration detection signal is received from the vibration sensor . <EOS>