PURPOSE: To perform stable motor control by setting the rotational speed of a motor to the narrow speed range including the stable speed range when it is separate greatly from the stable speed, and changing the speed range over to the one wider than the narrow speed range when it reaches the narrow speed range.

CONSTITUTION: Since the value f1, with which the speed range to operate the phase control system becomes relatively narrow, is given to the parameter fA, it judges that to be not f0-f1<f<f0+f1 at the start of the motor, etc., such as a C part. And a flag A is made 0 and f1 is given to fA. Then, each time a CTL signal is input, it outputs a fixed phase value P0 as a PWM signal. When the rotational speed f of the motor reaches the vicinity of the stable speed f0 and it judges that f0-f1<f<f0+f1, the flag A is made 1, and fA is changed into f2 larger than f1. Then, each time CTL signal is input within the range of f0-f2<f<f0+f2, the phase P calculated from the input time of the CTL signal is output from the PWM output circuit.