To provide a rotary throttle valve type diaphragm carburetor capable of easily controlling a fuel flow rate flowing from a carburetor to an engine, according to operation conditions of the engine.

A cylindrical valve chest 57 across an intake passage 16 is provided on the carburetor body 15 through which the intake passage passes. A cylindrical throttle valve 47 having a throttle hole 16 is rotatably and movably fitted into the valve chest 57. While a fuel supply tube 56 is projecting from a constant pressure fuel chamber to a throttle hole 47b, a needle 55 projecting from the throttle valve 47 is inserted and fitted into the fuel supply tube 56, so that air can be adjusted by the rotation of the throttle valve 47 and fuel rate can be adjusted by the elevation of the throttle valve 47. A first bypass passage extending from the inlet of the intake passage 16 to the valve chest 57, and a second bypass passage extending from the valve chest 57 to the outlet of the intake passage are provided on the sidewall of the carburetor body 15. A cutout 79 and cutout 78 which communicate to the first bypass passage and the second bypass passage respectively when the throttle valve 47 is fully open are provided on upper peripheral edge of the throttle valve 47.