To prevent fuel from being rich even when a carburetor is regulated by a driver and to prevent worsening of an exhaust gas component.

In this membrane type carburetor, a high speed fuel passage is provided to have one end communicated with a constant pressure fuel chamber and the other end opening to the venturi part 16a of an intake passage 16 through a fixed jet 46 to regulate a fuel amount, and a high speed fuel injection nozzle 51. In this case, the starting ends of first and second air passages A and B is vertically opened to the wall part, situated upper stream from a venturi 16a, of the intake passage 16 and the terminal ends of the first and second air passages A and B are connected to an intermediate part between the fixed jet 46 of the high speed fuel passage and the high speed fuel injection nozzle 51. An air regulation valve 61 to regulate a passage area is arranged in the first air passage A and a throttle 65 to regulate a passage area is located in the second air passage B.