A base station in a mobile communication system transmits and receives wireless signals with a user apparatus in a cell; in the case where the base station is connected to a communication network through a router, obtains an address of the router; periodically receives time information from a time information server through the communication network; stores in a memory unit the router's address and a current time indicated by the time information; and (1) in the case where a current-time-storing-is-stopped period is equal to or greater than a threshold value, or (2) in the case where the current-time-storing-is-stopped period is less than the threshold value and is equal to or greater than another threshold value, and an address just received from the router is different from the address stored in the memory unit, transmits a control signal for stopping the transmission of the wireless signal.