To stabilize a discharge rate by opening a pulsation pressure taking- out port to a position upper than a throttle hole of a rotary throttle valve at the time of idling and to a position of a downstream side from the rotary throttle valve, in a rotary throttle valve type diaphragm carburetor provided with a fuel pump driven by pulsation pressure of an engine.

At the time of start of engine, intake air is sucked to an engine through an intake passage 23 and a throttle hole 24a of a rotary throttle valve (a throttle valve) 24, and simultaneously, fuel in a constant pressure fuel chamber 34 is injected from a fuel injection hole 31a by a rate according to an opening of the throttle valve 24. An taking-out port 37a of pulsation pressure of an engine used as a driving source of a fuel pump A for forcibly feeding fuel by vertical displacement of a film 35 is opened to a position of a downstream side from the throttle valve 24 of the intake passage 23, and to an upper position of the downstream side from the throttle hole 24a of the throttle valve 24 at the time of idling, and to the position opposed to the throttle hole 24a at the time of idling. Accordingly, pulsation pressure of the engine is led to a fuel pump A without being influenced by fuel, and discharge performance is stabilized.