A comparator (12) for compensating the pulse width of the drive control signal of a power amplifier (54) by using a sense signal of an offset voltage generated between the nodes (H, I) of the power amplifier (54) is provided to enlarge the time width from the OFF operation of a pair of switching elements (Q1, Q4) to the ON operation of a pair of switching elements (Q2, Q3), thereby preventing the inconvenience that the switching elements (Q1, Q2) simultaneously turn on and suppressing a through-current. The pulse width for turning on the switching elements (Q1, Q4) is so corrected as to be wider or narrower than the pulse width for turning on the switching elements (Q2, Q3), thereby adjusting the intensity of an impressing voltage and canceling the offset voltage.