The invention relates to a drive device and a drive method for a brushless motor. In a drive method in which a switching timing of energization patterns is detected by comparing a pulse induced voltage of a non-energized phase with a voltage threshold, when the pulse induced voltage does not reach the voltage threshold, the pulse induced voltage at an angle of switching the energization patterns is measured. Discrimination between a demagnetization failure and a lock failure is performed based on whether the measured pulse induced voltage satisfies a predetermined condition or not. When it is the demagnetization failure, motor control continues after changing the voltage threshold. When it is the lock failure, the motor control stops.