The invention relates to a mobile station apparatus in a communication system. The communication system comprises a plurality of base stations for providing mobile communication service within a covered communication area. The mobile station apparatus comprises a memory for recording a predetermined access control class and determining which base station the mobile station apparatus is allowed to camp on for conducting mobile communication service, a battery for providing electrical power, a control program, and a power detector for detecting the current power of the battery, so that when the power detector detects that the power is lower than a predetermined low power watermark, the power detector issues a trigger signal. When the control program receives the trigger signal, the control program overwrites the predetermined access control class, so as to allow the mobile station apparatus to conduct mobile communication service through other base stations in the communication area.