rotation controlling the brushless motor 2 the first drive signal generating means 34a has a timer which in this case comprises pmw signal generating means see fig1 b to 15 g for generating as first drive signals u3 x3 v3 y3 w3 and z3 six pwm signals for turning on off as desired six transistors u1 x1 v1 y1 w1 and z1 of the first drive circuit 5 for rotation controlling the three phase induction motor 15 the microcomputer 34 obtains intersections between the modulated waves and the fundamental waves u1 v1 and w1 at a half period tf see fig1 a to set tu1 tv1 and tw1 up to the respective intersections to 14d 14e and 14f then the microcomputer 34 sets the half period tf of the modulated wave to the memory 34c and sets the obtained values tu1 tv1 and tw1 to the memories 34d 34e and 34f the timer counter 34h is reset to cause it to start and simultaneously therewith three signals u1 v1 and w1 outputted from the comparator means 34g take a l level in the comparator means 34g the counted value of the timer counter 34h is compared with the values tu1 tv1 and tw1 of the memories 34d 34e and 34f when they coincide the respective output signals are inverted when the counted value of the timer counter coincides with the value of the memory 34c at the half period tf of the modulated wave.