A motor-assisted turbocharger is composed of a turbine driven by energy of exhaust gas, a compressor rotated by the turbine and a rotary electric machine for assisting rotation of the compressor. The turbine, the compressor and the rotary electric machine are connected to each other by a common rotating shaft. A polygon nut having magnetic member facing fixed magnetic sensor is connected to an axial end of the rotating shaft. A magnetic field formed between the magnetic member and the magnetic sensor changes according to rotation of the rotating shaft. A rotational speed and a rotational position (or angle) of the compressor are detected based on the changes in the magnetic field. Operation of the turbocharger is electronically controlled based on the detected rotational speed and the rotational position.