To provide a keyless entry system using a cellular phone with increased security effect.

An IC card comprises a storage means in which a secret key is stored, a random number generating means, and an electronic signature data generating means. The cellular phone comprises a means for reading random numbers and electronic signature data from the IC card and a means for transmitting the read random numbers and electronic signature data to a locking/unlocking controller. The locking/unlocking controller comprises a means for receiving the random numbers and electronic signature data, a lock control means controlling a locked state and an unlocked state in starting an engine and opening/closing a door, a storage means in which an electronic certificate to which a public key used in pairs with the secret key is registered, an electronic signature data verification means collating a hash value obtained by decoding the received electronic signature data with the public key registered to the electronic certificate with a hash value calculated by using the received random number, and a means for transmitting signals for unlocking to the lock control means when the electronic signature data is verified.