Between an IC card (1) and a electronic ticket server (2), upon purchasing an electronic ticket a public key cryptosystem is employed for a mutual authentication to keep a strict security, and a shared secret between the electronic ticket and a ticket collecting machine is sent on a secure channel as well as the electronic ticket. Upon usage of the electronic ticket, the IC card (1) and the ticket collecting machine (6) mutually judge whether they carry out the mutual authentication by using a public key cryptosystem or a symmetric key cryptosystem. When they determine to use the symmetric key cryptosystem, they carry out the mutual authentication by using the shared secret exchanged beforehand. When they determine to use the public key cryptosystem, they carry out the mutual authentication by using the same method as that used upon purchasing the electronic ticket. This method of mutual authentication can dynamically switch cryptosystems for cryptographic communication between the IC cards or between the IC card and a tamper resistant device having an equivalent function to an IC card installed in related servers or the ticket collecting machine, based on their judgment of the types of bearers, the required degree of security and the required processing speed.