Electronic key registration system

An offline immobilizer ECU (6) reads an encryption key generation code (SC-A3) from an offline additional electronic key (2c) and generates an electronic key encryption key for the offline additional electronic key (2c) using the encryption key generation code (SC-A3) and a communication subject key encryption key (VK-A) held by the immobilizer ECU (6). The immobilizer ECU (6) stores, in a memory (69), the generated electronic key encryption key (VK-A) and a key ID code (KID-3) that is read from the offline additional electronic key (2c).