A base station used in a mobile communication system, includes: a controller configured to cause a user equipment to transition to a specific state in which a paging area is configured in the user equipment from the base station, perform a paging for the user equipment by the base station with another base station that are correspond to the paging area, when the base station receives a non-access stratum signaling from a core network to the user equipment, and transmit a failure notification from the base station to the core network in response to failing to the paging. The failure notification comprises an identifier for identifying the user equipment, and information indicating a cause of failure of the paging.