To obtain smooth accelerating operation of an engine by gradually increasing pressure in a constant pressure fuel chamber and increasing a fuel amount at the time of abrupt acceleration of the engine.

The inner space of a membrane type carburetor is divided into a constant pressure fuel chamber 25 and an atmospheric pressure chamber 30 by means of a membrane 29. the membrane 29 is energized to the side of the atmospheric pressure chamber by the force of a spring 24. Fuel in the constant pressure fuel chamber 25 is sucked into an intake passage 16 through a fuel nozzle 56. In such a membrane type carburetor, the atmospheric pressure chamber 30 is opened to atmosphere through a throttle 39, and connected with a pump chamber 73 of an acceleration pump 70. A negative pressure operation chamber 77 of an acceleration pump 70 is connected to a downstream side portion 15c of a throttle valve 47 on the intake passage 16 through a pressure regulator 42.