A communication system includes a base station and a mobile terminal device. The base station includes a detection information notifying unit. The detection information notifying unit notifies, using a predetermined control channel, the mobile terminal device of detection information that includes a power-saving time that indicates a time that elapses before the base station enters a power-saving state. The mobile terminal device includes a detecting unit and a region information notifying unit. The detecting unit, when being notified of the detection information by the detection information notifying unit, detects a region in which a mobile terminal device is not capable of communicating with any base station based on the power-saving time included in the detection information. The region information notifying unit notifies a given base station of information on the region detected by the detecting unit.