There is provided a method for synchronization between one or more base stations to provide broadcast multicast service, including: receiving a session start request message, and extracting a service start setting time set to start broadcast service from the session start request message; comparing the service start setting time to a modification check time being a check time which is later than and closest to the service start setting time, among check times at which it is determined whether or not broadcast service has changed, the check times having a predetermined time interval; deciding, when the modification check time is later than the service start setting time by a predetermined internal slack time period or more, the modification check time as a service start time at which broadcast service starts; and transmitting information indicating the service start time to the base stations.