To enhance the reliability of communication data transmitted to a server 2 from a user terminal 5 through networks 4, 3, 6, and 1.

The communication data received by the server 2 are transferred to another server 6, and receipt time in the other server 6 is made to reply with the communication data (time authentication). The user terminal 5 encrypts the communication data by using a session key created from a common key and random numbers. The random numbers and the encrypted result are transmitted to the server 2 with the communication data, and the server 2 also performs the same processing by using the common key (tampering prevention). In the user terminal 5, the communication data are encrypted with the created session key, the session key is encrypted with a secret key and transmitted. The server 2 decrypts the session key and the communication data with a public key, and decides the coincidence between the signature encrypted with the communication data and the received signature (tampering prevention, improvement of data secrecy).