To provide a method for security for a communication network that can ensure security of communication by mutual authentication between a service provider and a subscriber.

This security method includes a process where a service provider initially registers a subscriber via an operator and a process where a communication session 30 is executed between the subscriber and the service provider. In the initial registration process above, authentication data (Device ID, R1: 22) and (login, mdp: 23) are exchanged in on-line or off-line, then an encrypted channel is set up at the start of each session after the mutual authentication including an encryption function and an encryption key Kses 33 is calculated without transmitting a secret element on a network after that. This method is applied preferably to inter connection between a global system for mobile communications GMS and the Internet or a similar network.