Electronic key system for vehicle

An electronic key system for a vehicle which allows start of a host vehicle when an electronic key of the host vehicle is authenticated, the electronic key system comprising:

• an electronic key information acquisition unit configured to acquire movement locus information calculated by integrating an acceleration of the electronic key;
• an opening/closing detection unit configured to detect opening/closing of a door of the host vehicle;
• a taking-out detection unit configured to detect that the electronic key has been taken out of the host vehicle on basis of the movement locus information acquired by the electronic key information acquisition unit when the opening/closing detection unit detects that the door of the host vehicle is open after a power source of the host vehicle starts and further detects that all doors of the host vehicle are closed;
• and an alarm unit configured to transmit information indicating that the electronic key has been taken out of the host vehicle to said electronic key through short-range wireless communication, and induce the electronic key that received the information indicating that said electronic key has been taken out of the host vehicle to provide a warning.