To detect revolutions of a DC motor without giving a mechanical load to the motor by removing a miscount from a pulsating current flowing in the motor, and detecting only a fundamental wave proportional to the speed.

A pulsating current flowing in the DC motor M is detected by a current transformer T, and amplified by a band pass amplifier A to become a predetermined amplitude. Comparators C1, C2 are provided, and threshold values e1, e2 are set. A miscount is removed by a set-reset flip-flop F of a next stage, and only a fundamental wave proportional to a rotational speed is detected. With this arrangement, the revolutions can be purely electrically detected without necessity of a mechanical coupling.