PURPOSE: To achieve the enhancement in accuracy for controlling minute quantity of air at the time of idling, by deviating eccentrically the center of a turn throttle valve provided in an intake air passage from the longitudinal axis line of the intake passage.

CONSTITUTION: A turn throttle valve 15 is provided inside the intake air passage 11 penetrating a carburetor 10. The center A of this turn throttle valve 5 is arranged so as to be eccentrically deviated from the longitudinal axis line B-B of the intake air passage 11. Thus, in a minute opening of the throttle valve, the intake air quantity is controlled only by the plate-shaped throttle valve part 15J on one side, and the control accuracy can be enhanced.