An electronic key system for an entry object such as a vehicle is comprised of a portable device and an in-object device. The portable device has a device body and an emergency key detachably accommodated in the device body. The emergency key has a transponder, which is separate from a transponder provided in the device body. The transponder is for verifying the authority of registration of an additional portable device, while the transponder is for verifying the authority of vehicle control. The device body is also capable of detachably accommodating an emergency key which has no transponder.