The present invention relates to a drive device for a brushless motor utilizing no position detector for detecting the position of a movable element of the motor.The drive device comprises a phase controlled loop for controlling an output of a voltage controlled oscillator (40) by supplying an electric power to motor driving coils (1, 2, 3), detecting the difference in phase between the power supply waveforms thereof and a counterrelectromotive force of the motor driving coils during a power supply interrupting period with the use of a phase difference detector (20) and inputting a detected phase difference signal to the voltage controlled oscillator (40), an oscillation frequency initializing means (70) and a driving transistor interrupting circuit (80).