NOVELTY - The device has multiple on/off switches that alternatively change a connection of a capacitor and a battery to a parallel connection, a series connection or an independent connection of the capacitor and the battery. The battery selectively supplies the electrical power to an electric supercharger of on-vehicle electrical equipment by a parallel connection circuit, a series connection circuit or an independent connection circuit of the capacitor and the battery, whose connections are alternatively changed by the on/off switches.    USE - Power supply device for electric supercharger of internal combustion engine such as multi cylinder petrol engine, diesel engine and rotary engine mounted in vehicle.    ADVANTAGE - The power supply device suppresses an adverse influence on the on-vehicle electrical equipment caused by voltage drop or voltage fluctuation which occurs due to a driving of the electric supercharger and suppresses the deterioration of the battery to realize long useful life of the battery. The power supply device stably supplies the electrical power to the electric supercharger. The power supply device for vehicle has capacitor that is charged and discharged in a short time compared to the battery. The compressed air is cooled down by an intercooler and the charging efficiency is improved. The connection is prevented from frequently changing around the threshold value. The current acceptance characteristics are improved, and the voltage fluctuation of the bus voltage is reduced. The series connection is contributed to loss reduction and the efficiency of the generator is improved. The battery power is aggressively suppressed from usage. The capacitor with good input and output characteristics is aggressively used as the power supply which supplies electrical power necessary for the electric supercharging, and the frequency in use of the electric supercharger is high since the electric supercharger is used. The inputs and outputs of the electrical power input from the battery are reduced, and a useful life of the battery is extended.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the petrol engine.    Electric motor (3)    Compressor impeller (4)    Throttle valve (10)    Intake manifold (11)    Petrol engine (16)