A disclosed mobile station capable of using a plurality of communication systems may reduce power consumption when located in a service area other than that of a communication system that is set during use as a single-mode mobile station. The mobile station has communication processors respectively supporting the two communication systems; a communication unit switching section, which sets communication to either of the two communication systems; and a communication switch selector, which controls the switchover of the set communication system. When the mobile station location is out of communication range for the set communication system, the communication switch selector carries out a search to determine if the mobile station is in the other communication system service area, and if the mobile station is inside the other communication system service area, sets the mobile station to a monitoring mode for regularly monitoring notice information from the base station of the above-mentioned other communication system without carrying out location registration processing.