The power-assisted supercharger (10) includes: a turbocharger (12) in which a turbine (12a) is rotationally driven by exhaust gas (7) from an engine (40), a compressor (12b) rotationally driven by the turbine compresses intake air (5) and supplies the intake air to the engine; an electric motor (14) connected to a rotary shaft (12c) connecting the turbine and the compressor, the electric motor to add rotational force to the turbocharger; and a motor controller (20) to control a drive current (I) to the electric motor required for an assist operation for the turbocharger, by open loop, based on an accelerator opening degree (A) of the engine and a rotational speed (NE) of the engine. In this power-assisted supercharger, a turbo lag can be reduced by itself, power consumption can be small even in a range in which the rotational speed is high, malfunctions or unnecessary power consumption can be prevented, a control system can be simplified, and the installation space and the weight thereof can be small.