After an output signal S4 is level-inverted, first and second shorting FETs 55, 56 as a level-inversion inhibiting circuit inhibit level-inversion so that the signal is maintained to the inverted state. Thereafter the inhibition of level-inversion is released, when the signal is subsequently level-inverted at a proper time according to a desired duty ratio of a PWM signal S1. Thus chattering can be prevented and thereby a PWM signal S1 of a stable duty ratio can be generated, even if the level of a reference signal S3 fluctuates due to a noise or the like during vehicle acceleration, for example.