PURPOSE: To improve an engine speed control over the total region, by arranging a throttle valve, which moves being associated with an accelerator control, of a carburetor and an intake control valve in series in an intake passage and maintaining the engine speed to a predetermined value, when an engine is in idle, and to a rated speed, when the engine is in accelerator control, by the intake control valve.

CONSTITUTION: A throttle valve 6, which moves being associated with an accelerator control part 7, of a carburetor 2 and an intake control valve 8, whose opening is adjusted by an actuator 11, are arranged in series in an intake passage 10. The actuator 11 is controlled by an electric control unit 5 which inputs a throttle valve position signal, engine speed signal, electric load signal and a mechanical load signal. The throttle valve 6 is controlled so as to maintain an engine speed to a predetermined value for a change of load, in the time of idling, further to fully open, in the time of acceleration, thereafter to obtain a rated speed.