PURPOSE: To carry out injection supply into an intake passage and restrain the thinning of mixture by opening a fuel quantity increase passage by actuating a control valve by means of a control signal within the range of an engine rotation number being determined and also within the range of intake pipe inside negative pressure.

CONSTITUTION: The effective opening area of an intake passage 2 that runs through a carburetor main body 1, is conducted mechanically 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. Also, a fuel quantity increase passage 20 whose one end is connected to a fuel inflow passage 17 and whose other end is connected to the intake passage 2 of the carburetor 1 that includes an intake pipe, and a control valve 21 or the like that conducts the opening/closing control of the fuel quantity increase passage 20 by means of a control signal from a control circuit and at the same time conducts the control of a flowing fuel quantity, are equipped. The fuel quantity increase passage 20 is opened by actuating the control valve 21 by meansof the control signal within the range of an engine rotation number being determined and also within the range of determined intake pipe inside negative pressure. As a result, the thinning of mixture at the time of high opening operation can be restrained.