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.