PURPOSE: To detect the revolution of a DC motor through a detector circuit with a simple constitution very easily and accurately by obtaining very simple output pulses from the auxiliary brush of the DC motor without lowering the driving efficiency of the DC motor, because the auxiliary brush of the DC motor is in slide contact with only a commutator segment.

CONSTITUTION: An extension 23a, which is extended in the direction of the revolving shaft of a DC motor 20, is formed in one commutator segment 23 out of commutator segments 21-28 constituting the DC motor 20; and an auxiliary brush 43 is arranged so as to be in slide contact with only the extension 23a.