The present invention relates to a motor drive device and a phase current detecting method for a three-phase brushless motor, and more particularly to a technique of detecting each phase current through A/D conversion that is carried out once per PWM carrier cycle, by use of a current sensor for detecting a DC bus current of an inverter. The control unit of the present invention calculates a phase current for one phase, an absolute phase current value of which becomes maximum at a detection timing that is set once per PWM control cycle, from an output of the current sensor, and estimates phase currents for the remaining two phases based on a peak value of a phase current detected at each detection timing, a motor angle, and a phase shift of an actual phase current waveform relative to the motor angle.