PURPOSE: To restrain the thinning of air-fuel mixture by opening a fuel quantity increase passage by actuating a control valve by means of a control signal with a throttle valve opening being more than a fixed opening and also within the range of intake pipe inside negative pressure that is determined within the range of the opening of the throttle valve.

CONSTITUTION: A carburetor 1 at which the effective opening area of an intake passage 2 is conducted with opening/closing control by means of a throttle valve 9 and at the same time a venturi portion 11 is formed at the intake passage 2, and a throttle valve opening sensor 27 to detect the opening of the throttle valve 9, are equipped. Also, a control valve 21 or the like that conducts the opening/closing control of a fuel quantity increase passage 20 and at the same time conducts the control of a fuel quantity that flows in the fuel quantity increase passage 20, is equipped. The fuel quantity increase passage 20 is opened by actuating the control valve 21 by means of a control signal from a control circuit 30 with a through valve 9 opening being more than a fixed opening and also within the range of intake pipe inside negative pressure that is determined within the range of the opening of the throttle valve 9. As a result, the thinning of mixture at the time of the middle and high opening operation of an engine can be restrained.