This electronic key system 1 includes: an electronic key which is enabled to engage in radio communication with a vehicle on condition that a vibration is detected by a vibration detection part; and the vehicle which has an on-board control part for starting an engine on condition that the radio communication with the electronic key is established, and a push switch installed in the vehicle is operated. The on-board control part causes the vehicle to be vibrated by the engine on condition that the push switch is operated.