PURPOSE: To simplify a circuit without necessity for complicated process using an integration circuit, etc., by comparing phase voltages of armature windings of a sensorless brushless motor with each other, and obtaining a rotor position signal.

CONSTITUTION: When a motor 2 starts, phase voltages U, V, W present at armature windings 3u, 2v, 3w are applied to comparators 6a, 6b, 6c through filters 9u, 9v, 9w. The comparators 6a, 6b, 6c obtain differences of output voltages Vu, Vv, Vw, and output voltages V1, Vb, Vc. The voltages Va, Vb, Vc respectively correspond to predetermined angular positions of a rotor 4 to become rotor position signals of the motor 2. A controller 5 applies it to a switching signal generator 54 through contacts 58a, 58b, 58c to generate a switching signal. Since a commutation circuit 1 is controlled by the switching signal, it is ordinarily stably operated at a predetermined synchronizing speed.