A power-assisted supercharger comprising: a turbocharger in which a turbine is rotationally driven by exhaust gas from an engine, a compressor rotationally driven by the turbine compresses intake air and supplies the intake air to the engine; an electric motor connected to a rotary shaft connecting the turbine and the compressor, and the electric motor to add rotational force to the turbocharger; and a motor controller to control a drive current to the electric motor required for an assist operation for the turbocharger, by open loop, based on an accelerator opening degree of the engine and a rotational speed of the engine, wherein the motor controller includes: a steady instruction value arithmetic unit to calculate a steady instruction value required for a steady operation of the turbocharger, from the accelerator opening degree of the engine; an acceleration instruction value arithmetic unit to calculate an acceleration instruction value required to an acceleration operation of the turbocharger, from an accelerator opening degree change rate (dA/dt) of the engine; and a drive current arithmetic unit to calculate the drive current to the electric motor, from the steady instruction value and the acceleration instruction value, and the drive current arithmetic unit to output the drive current, characterized in that the drive current arithmetic unit includes: an adder to calculate a combined instruction value by adding the steady instruction value and the acceleration instruction value; a limiter to generate an output instruction value by limiting the combined instruction value to an upper limit thereof or less; and a motor driver to calculate the drive current to the electric motor based on the output instruction value, and to output the drive current. a turbocharger in which a turbine is rotationally driven by exhaust gas from an engine, a compressor rotationally driven by the turbine compresses intake air and supplies the intake air to the engine; an electric motor connected to a rotary shaft connecting the turbine and the compressor, and the electric motor to add rotational force to the turbocharger; and a motor controller to control a drive current to the electric motor required for an assist operation for the turbocharger, by open loop, based on an accelerator opening degree of the engine and a rotational speed of the engine, wherein the motor controller includes: a steady instruction value arithmetic unit to calculate a steady instruction value required for a steady operation of the turbocharger, from the accelerator opening degree of the engine; an acceleration instruction value arithmetic unit to calculate an acceleration instruction value required to an acceleration operation of the turbocharger, from an accelerator opening degree change rate (dA/dt) of the engine; and a drive current arithmetic unit to calculate the drive current to the electric motor, from the steady instruction value and the acceleration instruction value, and the drive current arithmetic unit to output the drive current, a steady instruction value arithmetic unit to calculate a steady instruction value required for a steady operation of the turbocharger, from the accelerator opening degree of the engine; an acceleration instruction value arithmetic unit to calculate an acceleration instruction value required to an acceleration operation of the turbocharger, from an accelerator opening degree change rate (dA/dt) of the engine; and a drive current arithmetic unit to calculate the drive current to the electric motor, from the steady instruction value and the acceleration instruction value, and the drive current arithmetic unit to output the drive current, the drive current arithmetic unit includes: an adder to calculate a combined instruction value by adding the steady instruction value and the acceleration instruction value; a limiter to generate an output instruction value by limiting the combined instruction value to an upper limit thereof or less; and a motor driver to calculate the drive current to the electric motor based on the output instruction value, and to output the drive current. an adder to calculate a combined instruction value by adding the steady instruction value and the acceleration instruction value; a limiter to generate an output instruction value by limiting the combined instruction value to an upper limit thereof or less; and a motor driver to calculate the drive current to the electric motor based on the output instruction value, and to output the drive current.