A user authentication process when a portable terminal receives multimedia broadcasting requiring authentication is provided. The broadcasting is provided by a mobile communication system including a portable terminal capable of a user authentication process if a message indicating that broadcasting requires the authentication process is received while using the broadcast service, the portable terminal provides the broadcast service if the authentication process is successful, and stops the broadcast service if the authentication process fails; and a multimedia broadcast server for transmitting to the portable terminal the message, which indicates that the broadcasting requires the authentication process, when the broadcasting requires the authentication process while the broadcast service is being provided to the portable terminal.