A mobile device includes a receiver receiving a radio wave from a wireless communication device, a transmitter transmitting an authentication signal when the radio wave that is received requests transmission of the authentication signal, a vibration detector detecting whether there is vibration, and an operation controller bringing the mobile device into a communication enabled state where the receiver and the transmitter are in operation when the vibration is detected, and bringing the mobile device into a communication disabled state where at least one of the receiver and the transmitter is stopped when the vibration is not detected. When the radio wave is received in the communication enabled state, the operation controller brings the mobile device into a communication enabled continuous state where the receiver and the transmitter are in operation even if the vibration is no longer detected.