To provide a rotational speed detector that prevents the lowering of the detection accuracy of a rotor due to shaft runout.

The rotational speed detector 10 for detecting the rotational speed of rotors 41, 51, 8 and 6 of a turbocharger 1 with an electric motor includes a gap sensor 12 for detecting a distance between a compressor blade and the sensor 12, and a rotational speed computing means for computing the rotational speed of the rotors 41, 51, 8 and 6 based on the appearing time of a peak value appears in an output signal from the gap sensor 12 every time the rotational position of the compressor blade faces to the gap sensor 12.