To provide a carburetor to form a fuel system in a single system constitution of which is simplified, perform the stable feed of fuel in a proper air-fuel ratio, and besides the height be low.

A fuel injection nozzle 7 opened to a spot situated downstream from a butterfly throttle valve 5 is communicated with a control pressure chamber 11 through an air-fuel ratio passage 10, and a fuel passage 12 and an air introduction passage 14 are connected to the control pressure chamber 11. The throttle areas of the air-fuel mixture passage and the air introduction passage are set to a value at which a pressure in the control pressure chamber enters a region where air compression is prevented from occurring. By controlling fuel fed from the control pressure chamber by a control valve 23 interlocked with the butterfly throttle valve, air-fuel mixture the air-fuel ratio of which is kept at a constant value is generated in the control pressure chamber. When compressive air-fuel mixture is fed to an intake passage 3 and mixed in air controlled by the butterfly throttle valve, compressive fluids are mixed together in the intake passage and air-fuel mixture fed to an engine is brought into a constant value at a whole rotation area. This constitution forms a fuel system in a single system.