To hold a throttle valve at a position where air and fuel are increased in quantity by an unskilled simple operation so as to securely start an engine at a low temperature.

In a carburetor, a cam piece 23 disposed in a shaft member 22 is provided at the lower surface of a throttle valve lever 12 for transmitting an acceleration operation to a throttle valve 4. With manual rotation of the shaft member 22, the throttle valve lever 12 is swirled while being pushed up from an idle position by the cam piece 23, so that the throttle valve 4 is moved to a position where air and fuel are increased in quantity. The cam piece 23 is held at the quantity increasing position in contact with the throttle valve lever 12, and then, is rotated by a return spring 26 to be returned to an inoperative position when the throttle valve lever 12 is separated by the acceleration operation.