PURPOSE: To perform the long life of a carbon brush by decelerating a rotating speed when the brush is abnormally worn and rotating it at a high speed after the recovery of commutation by monitoring commutation conditions.

CONSTITUTION: A signal proportional to a rotating speed is detected by a rotating speed detection sensor 9, and converted by a F/V converter 10 to a DC level. A signal detected by a spark sensor 7 is converted by an F/V converter 15, superposed on the signal from the sensor 9, and used as a rotation signal. That is, the maximum speed is limited ideally unless the signal from the sensor 7 becomes zero. Thus, a carbon brush can become adapted to a commutator in the best state to realize a preferable sparking state, thereby attaining a longer life of the carbon brush.