1. A base station used in a mobile communication system, comprising:a controller configured to,
cause a user equipment to transition from a RRC (radio resource control) connected state to a specific state in which a paging area is configured in the user equipment from the base station,
transmit a resume identifier used for resuming a RRC connection of the user equipment, to the user equipment when causing the user equipment to transition from the RRC connected state to the specific state,
perform a paging for the user equipment by the base station with another base station that corresponds to the paging area, when the base station receives a non-access stratum signaling from a core network to the user equipment,
transmit, to the another base station, a paging request message for paging the user equipment, the paging request including the resume identifier and information indicating DRX configuration configured from the base station to the user equipment,
receive, from the another base station, first information indicating that the another base station has failed in paging the user equipment, and
transmit, to the core network, a failure notification indicating the failure of the paging.