To prevent a step-out or other troubles by detecting the exact position of a rotor of a brushless motor and making a proper changeover of excitation possible.

When switching the DC power from a DC power supply by switching elements of an inverter circuit 3 and then applying the switched DC power to a brushless motor 4, a control circuit 10 outputs a drive signal for chopping the switching elements of the inverter circuit 3, a chopping signal, and a control signal (changeover of the chopping) to chop the switching elements. Meanwhile, the control circuit 10 detects the position of a rotor 4a of the brushless motor 4 from position detection signals A, B, C output from a position detecting circuit 5 and then changes over the excitation of armature windings of the brushless motor 4, based on the detected position of the rotor 4a. In the position detection, the position detecting time intervals are counted. Then, a next position detecting time is estimated based on the position detecting time intervals, and the on-state time and off-state time of the chopping signal near the estimated position detecting time are temporarily varied to make the induced voltage and the reference voltage cross each other and thus a correct position detection is made.