A printer generates a public key with a passphrase containing at least printer position information and a random number by a public key cryptography, and holds this random number. A print client previously acquires the public key from the printer. When transmitting print data to the printer, the print client encrypts the print data in advance with the acquired public key and transmits the encrypted print data as print transmitting data. The printer which has received the print transmitting data generates a private key with a passphrase containing at least printer position information and the held random number by the public key cryptography. The printer then decrypts the received print transmitting data with the private key. Consequently, the security of the print data transmitted from the print client to the printer via a network is ensured.