An immobilizer ECU transmits a vehicle ID code and a SEED code, which is read from an electronic key, to a data center online. The data center generates an encryption key from the received SEED code and a first logic, and generates a further SEED code from the encryption key, the vehicle ID code, and a second logic. The immobilizer ECU obtains the further SEED code online from the data center, generates the encryption key from the obtained further SEED code, the vehicle ID code, and the second logic, and stores the encryption key.