PURPOSE: To miniaturize the title device and improve efficiency thereof by detecting induced voltage from the natural point of a stator coil and generating a pulse signal synchronizing with the revolution of a motor by the detection of induced voltage.

CONSTITUTION: A synchronizing-pulse generating circuit 7 compares induced voltage detected from the neutral point of a stator coil and an output from a voltage dividing circuit dividing the input voltage of an inverter circuit to half, and each outputs pulse signals by the rise and fall of the output. A synchronizing-pulse discriminating circuit 9 discriminates the polarity of induced voltage by the pulse signals and an output signal from a drive circuit 8, and outputs the polarity of induced voltage to an ignition pulse generating circuit 10. An inverter 4 controlled through the drive circuit 8.