A contactor failure determining method in includes outputting a control signal for switching a positive-side contactor provided between a positive electrode of a multi-cell battery and a positive electrode of a load to which the multi-cell battery supplies electric power to an open state from a state in which a control signal for controlling the positive-side contactor and a negative-side contactor provided between a negative electrode of the multi-cell battery and a negative electrode of the load to a closed state and a precharge contactor provided between the multi-cell battery and the load to an open state is output, and as a first fixing determining step, determining a presence of fixing of the precharge contactor based on a degree of voltage drop of a voltage applied to the load after the control signal for switching the positive-side contactor to the open state is output.