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 mobile communication system comprises: a cellular communication terminal that is a user terminal that performs the cellular communication; a D2D communication terminal that is a user terminal that performs the D2D communication; and a base station that assigns a radio resource that is used by the cellular communication terminal for the cellular communication. The base station transmits assignment information indicating a radio resource that may possibly be used by the cellular communication terminal from among D2D radio resources available in the D2D communication to the D2D communication terminal.