The purpose of the present invention is providing a supercharging system which uses both of a mechanism such as VN or the like and an electric motor generating an assist force while making the supercharger operate smoothly when the assist by the electric motor is stopped. The system controls the electric motor in a feedback manner so that enough assist force is generated (FIG. 2A and FIG. 2B) while controlling the VN in an open manner (FIG. 2D) until status of the supercharger reaches target status (time t1) when an accelerator requirement arises in a low revolution region. The control of the electric motor is changed to an open control, and the control of the VN is changed to a feedback control, respectively, at time t1. The open control of the electric motor is continued so that necessary complement torque occurs until time t2. After time t2, the system maintains the target status only by the feedback control of the VN.