To provide a mutual authentication system which is not required to erase master key when a slave device is replaced. The storage part stores a temporary key which is key data used temporarily and a master key which is key data used for authentication. The storage part stores the temporary key. The key confirmation unit inquires whether the slave device stores the master key. The key confirmation response unit confirms whether the master key has already been stored in the storage part for an inquiry from the master device and responds. The key introduction unit encrypts the master key by using the temporary key and transmits to the slave device. The key storage unit decrypts the encrypted master key by using the temporary key and stores in the storage part. The main authentication unit and the sub-authentication unit authenticate with each other by using the master key.