A network device that is used in a mobile communication system that supports cellular communication in which a data path passes through a core network, and D2D communication that is direct device-to-device communication in which a data path does not pass through the core network, comprises: a control unit that sets a D2D radio resource that is ensured for the D2D communication from radio resources be available to the mobile communication system. The control unit changes an amount of the D2D radio resource such that the D2D radio resource reaches an appropriate amount.