Vehicular electronic key system

A vehicular electronic key system includes an on-vehicle apparatus which is communicable with an electronic key when the electronic key is in a predetermined area outside of a vehicle. The on-vehicle apparatus generates alarm when a door lock operation is executed from the outside of the vehicle and when the on-vehicle apparatus is not capable of communicating with the electronic key. Further, on-vehicle apparatus generates alarm when a key ID does not corresponds with an apparatus ID. Therefore, the electronic key system can firmly prevent the electronic key from being mislaid in the vehicle.