PURPOSE: To reduce the ununiformity of torque in case of driving with 1∼2-phase excitation and attenuate oscillation and noise, by switching exciting current according to phase fluctuation.

CONSTITUTION: From a LOGIC A 10, the output of control signals S1, S2, S3, S4 for driving each phase exciting phase Φ1, Φ2, Φ3, Φ4 in order on specified timing is generated. The control signals S1∼S4 are transferred also to a LOGIC B 13. From the LOGIC B 13, current value change-over signal S5 is transferred to a driver 11. Accordingly, the set value of exciting current is changed accord ing to phase fluctuation so that the exciting current at the time of a two-phase mode may be the exciting current at the time of a one-phase mode which is multiplied by approx. 0.6∼0.8.