PROBLEM TO BE SOLVED: To shorten connection time by omitting mutual authentication procedures at a redundant IMS level in a mobile communication network in which IP-CAN and IMS are provided within the same domain, and to ensure required and sufficient security by providing simple authentication procedures. SOLUTION: When a secure communication channel is established by IP-CAN, the corresponding relationship between the user ID of a terminal unit 1 and a user IP address is notified to an SIP server 3 for providing IMS to the terminal unit 1 (step S106'). The notified user ID and user IP address are stored (step S107). The user ID included and user IP address included in a message received from the terminal unit 1 is collated with the stored user ID and user IP address, thus omitting the mutual authentication procedures between the terminal unit and IMS executed in registration processing. COPYRIGHT: (C)2007,JPO&INPIT