PURPOSE: To improve drivablity by controlling the opening of a flow control valve with an engine rotating number for air-fuel mixture to be at a preset ratio when a throttle valve is fully open and an engine rotating number is below a preset number.

CONSTITUTION: When a throttle valve is fully open and an engine rotating number is below a preset number, an air bleed control valve (flow control valve ABCV) 8 is open at an opening corresponding to an engine rotating number at the time of the closed condition. In this way, the amount of suction air is increased via an air bleed passage 7, and an air-fuel ratio is changed from a riched condition to a leaned condition and therefore improved in deviation to the riched condition with an engine rotating number. An air-fuel ratio is thus as preset when conditioned for a mainbody and equally set from low rotation to high rotation.