Power supply control device of the electric supercharger

An on-off switch is provided between a first battery acting to supply an electric power to on-vehicle electrical equipment and a second battery acting to supply an electric power to an electric supercharger. In the case that charge voltage of the first battery is smaller than a predetermined value and operating power of an electric supercharger 3 is larger than a predetermined value, the on-off switch is opened to suppress electrical effects on the on-vehicle electrical equipment. In other cases, the on-off switch is closed and the second battery is charged by a generator and the first battery. As a result, it is possible to suppress adverse effects on the on-vehicle electrical equipment due to voltage drop or voltage fluctuation of a power source occurring by driving the electric supercharger and thus to make a stable power supply.