To carry out stable fuel supply at a proper air-fuel ratio and made a fuel system in a single system simple in constitution.

Air-fuel mixture of a constant air-fuel ratio is provided by mixing air and fuel by a control pressure chamber 13 to be controlled in negative pressure not causing air compression. Thereafter, this air-fuel mixture and air measured in a main intake throttle part 8 are mixed in a main intake passage on the downstream of the main intake throttle part 8. As this air-fuel mixture and the air are both compressed fluid, the air measured by the intake throttle part 8 and the air-fuel mixture blown out of the control pressure chamber 13 are in proportion to each other, and an air-fuel ratio of the air-fuel mixture formed in the main intake passage 3 becomes constant in the whole driving region. Consequently, it is possible to carry out stable fuel supply at a proper air-fuel ratio and to make a fuel system in a single system simple in constitution.