A fixed subscriber unit includes a voltage measuring unit which acquires a DC input voltage value to be applied to a power circuit, and a comparator/judging unit which compares a preset DC voltage with the DC voltage measurement and judges whether the power circuit is driven by the DC voltage from an AC/DC adaptor or by the DC voltage supplied from a charge battery. When the power circuit is driven by the charge battery, the available operation time of the fixed subscriber unit is calculated by a calculation unit from the DC voltage measurement. The information thus obtained on the driving power state and/or the available operation time are aurally or visually notified to the user by a notification unit. The user thus can select an appropriate method of using the fixed subscriber unit according to the prevailing situation.