To improve acceleration property of an engine by improving a phenomenon of fuel becoming temporarily excessively lean in accelerating a membrane carburetor.

A membrane 25A divides a constant-pressure fuel chamber 26 from an atmospheric air chamber 27, and fuel in the constant-pressure fuel chamber 26 is supplied to an intake passage via a fuel supply pipe 18. The atmospheric air chamber 27 is opened to atmosphere via a throttling 70. An air pump 50 for discharging air interlocking with an opening of a throttle valve 12 is communicated with the atmospheric air chamber 27. The air discharged from the air pump 50 is introduced to the atmospheric air chamber 27 to push up the membrane 25.