PURPOSE: To stabilize a position detection signal to detect a rotor position of a brushless motor based on a terminal voltage.

CONSTITUTION: A terminal voltage (induced voltage) U is compared with a predetermined voltage (a value higher than 1/2 of the terminal voltage) by a first comparator 10d, a switch 10e is operated by this result to extract a voltage waveform of the voltage V lower than the predetermined voltage, and a spike voltage included in the voltage U is removed. This waveform is held as prescribed by a second holder 10f to obtain a voltage waveform of its envelope, the waveform voltage is compared with the voltage-divided 1/2 voltage by a second comparator 10h to extract a timing of a neutral point of the voltage U, an arc firing timing pulse delayed by a predetermined phase is obtained by a phase shifter 10i, terminal voltages V, W are input to second and third controllers 11, 22 to obtain an arc firing timing pulse. They are combined by a combining unit 13 to obtain a position detection signal of a rotor.