The invention relates to a brushless motor drive device that performs switching determination of energization modes of a three-phase brushless motor according to a sensorless method, and a drive method thereof. A pulse induced voltage induced in a non-energized phase of three phases by a pulse voltage is compared with a threshold to determine the switching timing of the energization modes. Here, by limiting a duty ratio of a voltage application to equal to or greater than a lower limit, a situation in which the pulse induced voltage is sampled while the pulse induced voltage is oscillating is avoided, and a situation in which the pulse induced voltage falls below a voltage detection resolution is avoided. As a result, determination of energization mode switching timing based on a pulse induced voltage can be performed more stably.