An object of the invention is to provide a technique for excellently controlling a supercharger equipped with an electric motor. A supercharging system for an internal combustion engine according to the present invention is provided with a supercharger equipped with an electric motor, where rotation of the rotary shaft of the supercharger is assisted by the electric motor, and control unit for controlling assisting force of said electric motor in such a way that the rotation speed of the supercharger becomes a target rotation speed. The control unit determines the target rotation speed of the supercharger based on any one of (1) the degree of opening of the accelerator, (2) the degree of opening of the accelerator and the rotation speed of the internal combustion engine, (3) the actual degree of opening of a throttle valve and the rotation speed of the internal combustion engine, and (4) the target degree of opening of the throttle valve and the rotation speed of the internal combustion engine, and controls the assisting force by the electric apparatus in such a way that the target rotation speed is attained.