To prevent breakdown of circuit elements, which is caused by a power supply pumping phenomenon, without stopping an operation.

When a detection circuit 25 detects that an absolute value of a charging voltage of a capacitor C1 is equal to or more than a predetermined value, switch means of constant current generating means is turned on, and a current value of a constant current I increases. Thereby, current values of a first current I1 and a second current I2 become large, and a ratio of those becomes small. Therefore, a ratio of charging speeds of capacitors C1 and C2 becomes small, and a modulation degree of a PWM signal OUT is reduced. Accordingly, a MOSFET 16 is not in an ON state continuously for a long time period, and as a result, a state in which the charging voltage of the capacitor C1 is equal to or more than the predetermined value can be avoided, and breakdown of the capacitor C1 can be prevented.