A mobile communication system supports cellular communication in which data communication is performed between a network and a user terminal and D2D communication in which data communication is directly performed among two or more user terminals. The D2D communication is performed by using a part of the uplink radio resources of the cellular communication. The uplink radio resources include a specific radio resource that is used in the transmission of a predetermined uplink signal in the cellular communication. The use of the specific radio resource in the D2D communication is regulated.