An electronic key system for a vehicle provides a warning corresponding to a traveling state if an occupant has dropped the electronic key while a vehicle is running. A control unit 14 of the vehicle outputs a request signal Sr to the electronic key 12 at transmission intervals T corresponding to the traveling state. An answer signal Sa caused by the request signal Sr from the electronic key 12 is detected. A warning lamp 68 is lit up at a point after the detection of the answer signal Sa stops. The transmission interval T of the request signal Sr is specified in a map 88 stored in the control unit 14, and is set short corresponding to the increase of the vehicle speed V, the acceleration/deceleration ‘a’, and the yaw rate Y.