To form a carburetor main body into a two-layer structure of an inner wall part having high heat conductivity and an outer wall part having low heat conductivity.

A throttle valve 2 having a throttle hole 3 is rotatably and liftably inserted in a cylindrical valve chamber 41 intersecting an intake passage 40 laterally perforating a carburetor main body 16. A throttle valve lever 13 is connected to a valve shaft 2a perforating a cover plate 15 for closing the valve chamber 41 upward from the throttle valve 2, and a cam mechanism is provided between the cover plate 15 and the throttle valve lever 13. A needle 4 supported on the throttle valve 2 is inserted in a fuel supply pipe 5 for supplying fuel in a constant fuel chamber 26 supported on the bottom wall part 19 of the valve chamber 41 to the throttle hole 3 to constitute the rotary throttle valve-type carburetor. An inner peripheral wall part 18 and a bottom wall part 19 of the valve chamber 41 which are made of a material having high heat conductivity are covered with an outer peripheral wall part 17 made of a material having low heat conductivity.