When the absolute values of the negative voltage Vnull and the positive voltage Vnull of a rectangular wave to be applied to the three phase coils of the motor driven using a rectangular-waved voltage are different, time T1 and time T2 for the rectangular wave are adjusted such that the area A representing the absolute value of a time integration value of the negative voltage Vnull and the area B representing the absolute value of a time integration value of the positive voltage Vnull becomes equal to each other. With this adjustment, the average voltage of the rectangular wave becomes of value 0, so that a DC component current is prevented from flowing into the three phase coils of the motor.