PURPOSE: To enable smooth starting by fixing the frequency of a rotary field to be produced in an armature winding and increasing the duty ratio properly with time.

CONSTITUTION: When six transistors in a semiconductor switching element group 2 are switched by means of a drive signal for the output signals 81-86 from a drive signal generating circuit 8, a rotary field is produced in an armature winding 4 and a magnet rotor 5 rotates to start a brushless motor 3. A synchronizing signal 100 is outputted with a predetermined frequency simultaneously with rising of a start command signal 151 and a duty ratio command signal 153 is outputted simultaneously with rising of the start command signal 151 such that the duty ratio increases with time along a line having predetermined inclination. Upon reaching to a level for maintaining stabilized rotation of the brushless motor 3, the duty ratio is maintained at that level.