A mobile communication system includes a mobile station (UE), a mobility management entity (MME), a serving GPRS (General Packet Radio Service) support node (SGSN), and a serving gateway (S-GW). The MME and the SGSN support an Idlemode Signaling Reduction (ISR) feature. The S-GW maintains all bearers in a case where at least one of the MME and the SGSN performs restart. The at least one of the MME and the SGSN pages the UE, upon reception of a Downlink Data Notification message regarding an international mobile-station subscription identifier (IMSI) from the S-GW. The UE re-attaches based on the paging.