PURPOSE: To start a brushless motor smoothly by increasing at least one of the frequency and the duty ratio of rotary magnetic field along a curve having differential coefficient decreasing with time while increasing the other with time along a line having predetermined inclination, thereby suppressing starting current and vibration of the motor.

CONSTITUTION: A synchronous signal 100 is outputted such that the frequency thereof increases with time along a line having predetermined inclination simultaneously with rising of a start command signal 151. A duty ratio command signal 153 is outputted such that the duty ratio increases along a line having predetermined inclination simultaneously with rising of the start command signal 151. When the frequency and the duty ratio of the synchronous signal 100 reach to such values as maintaining stable rotation of a brushless motor 3, they are stopped from increasing and maintained at constant values.