To improve security in an electronic key system and a key ID registration method.

An encrypted key ID code IDk' is stored in an immobilizer 20a. An ID code IDh1 for conversion which decrypts the encrypted key ID code IDk' is then stored in an electronic key 10. The immobilizer 20a utilizes the ID code IDh1 for conversion included in an ID signal to decrypt the encrypted key ID code IDk' and collates a decrypted key ID code IDk with a key ID code IDk included in the received ID signal. Only the encrypted key ID code IDk' is stored in the immobilizer 20a but the ID code IDh1 for conversion which decrypts the encrypted key is not stored therein. Therefore, even in the case where unauthorized analysis is performed on the immobilizer 20a, the key ID code IDk can be suppressed from being acquired.