In a mobile communication system, upon multicasting a service data through a common channel in a radio communication area, a user not subscribing is disabled a multicasted service data, and charge can be applied only for the subscribing user. As a generating method of a security key for applying security for the multicoated service data, in SGSN, the security key is generated corresponding to the multicasting service for security process. The multicasted service data applied security process can be transmitted through the common channel in the radio communication area between RAN and UE (terminal), and the service data cannot be decoded by the user who is not subscribing.