A method for minimizing overhead occurring caused by control information for encryption performed to protect MBMS data for an MBMS service in a mobile communication system. This method is implemented by distinguishing a case in which control information used for encryption is updated from another case in which the control information used for encryption is not updated, and transmitting different control information according to the distinguishment result. That is, when the control information used for encryption is not updated, only minimized control information is transmitted, and when the control information for encryption is updated, the entire updated control information is transmitted. Accordingly, the amount of control information transmitted along with MBMS data is minimized, contributing to an increase in the amount of MBMS data transmitted per unit time.