To accurately control engine speed including engine speed in an idle state with a simple and inexpensive structure.

A motor 160 for opening and closing a throttle valve 170 is PWM controlled. A target engine speed variation calculating section 220 calculates a target engine speed variation using an actual engine speed from an actual engine speed calculating section 210 and a target engine speed from a target rotation speed calculating section 260. A PWM pulse generating section 200b has a PWM micro pulse calculating section 240 and a PWM signal generating section 280. The PWM micro pulse calculating section 240 calculates at least one of a PWM duty correction value, a PWM duty correction value keeping time, and the number of PWM duty corrections in response to the target engine speed variation. The PWM signal generating section 280 transmits a PWM signal to the motor 160.