PURPOSE: To reduce the size and weight of a motor by a method wherein the driving of the rotation of a rotor is controlled by producing an output transistor switching signal, having the division of conduction with a predetermined electric angle and being smooth, from the generating voltage of an armature coil.

CONSTITUTION: Currents of transistors Q7-Q12 of I-V converter group 19 are compounded by an adder 16a in an AGC circuit 16 while the composite current W is compared with a set current by a comparator 16b in the AGC circuit 16 to control the current of the V-I converter group 15. Generated voltages (t), (u), (v), induced in an armature coil group 4, are converted into signals P1, P2, P3 corresponding to the change of the diode voltages as they are. In this case, respective base currents of the output transistors Q1-Q6 of a semiconductor commutator device 2 are truncated wave signals having the electric angle of 180° and, therefore, spike noise will never be generated in an armature coil group 4 and a filter circuit or the like is not necessitated.