The remaining battery capacity computation system has a remaining capacity computation apparatus, which is connected to an electrical equipment unit via a one line communication circuit. The connected electrical equipment unit transmits a detected current signal, that indicates current flowing in the battery, at a fixed transmission period to the remaining capacity computation apparatus via the communication circuit. The remaining capacity computation apparatus computes remaining battery capacity from the detected current, and outputs a signal, that indicates the computed remaining capacity, with prescribed timing to the communication circuit. The remaining capacity signal is transmitted to the connected electrical equipment unit and timed after completion of the output of one detected current signal and before commencement of the output of the next detected current signal.