To set the movement of the tip portion of a forcible operation lever driven by a throttle valve operation lever identical to that of the tip portion of a follower lever engaged with a sliding throttle valve.

A negative pressure chamber 14 is plotted in the upper side of a film 5 and an atmosphere chamber 13 is plotted in its lower side, the film 5 being held between the upper end part and the upper cover 4 of a carburetor main body 6. In a guiding opening 17 orthogonal to the intake path 33 of the carburetor main body 6, a sliding throttle valve 26 connected to the film 5 is supported so as to be vertically moved. A rotary shaft 1 for connecting a throttle valve operation lever 56 is disposed in the atmosphere chamber 13, and a forcible operation lever for engaging/disengaging a follower lever 8 rotatably supported by the rotary shaft 7 from the throttle valve operation lever 56 according to the opening degree of the sliding throttle valve 26 is axially supported by the throttle valve operation lever 56. A notch provided in the tip of the follower lever 8 is engaged with the pin 25 of a wall plate 22 in the side of the sliding throttle valve 26. On the upper surface of an upper end flange 26a, the film 5 and a pressing plate 10 are connected by bolts by laminating a reinforcing plate 50 integral with the wall plate 22 on its lower surface.