A base station according to a first aspect is used in a mobile communication system supporting a D2D proximity service which enables direct device to device communication. The base station comprises: a controller that determines whether to allow a user terminal to execute transmission/reception operations of a D2D synchronization signal to directly establish device to device synchronization; and a transmitter configured to transmit information indicating results of the determination to the user terminal. The controller performs the determination based on D2D range information indicating a permissible communication range of the user terminal in the D2D proximity service.