Electronic key system for vehicle

An on-vehicle apparatus communicates with an electronic key by wireless communication. The one-vehicle apparatus decides to unlock a vehicular door according to an identification result between first ID of the electronic key and unlock-operation ID of the on-vehicle apparatus, and decides to start an engine of the vehicle according to an identification result between second ID of the electronic key and engine-starting ID of the on-vehicle apparatus. The on-vehicle apparatus commands the electronic key to output third ID shorter in data length than the second ID, when the first ID corresponds with the unlock-operation ID. The on-vehicle apparatus permits starting the engine when the third ID corresponds with compacted data of the engine-starting ID.