The invention provides a specific battery detection means that specifies single batteries believed to be in an excess discharge state during operation of the electric motor for starting the engine, based on data for each battery pack detected prior to starting the engine. With the specific battery detection means, a change in voltage for specific single batteries is measured for a drop in voltage of a single battery by the discharge via the voltage detection means. The specific battery detection means is configured such that if the results of this measurement are outside a predetermined threshold value, the power supplied to the electric motor for starting the engine is limited.