To solve a problem that if a back surface of a venturi piston is made to be a recessed surface, fuel pools in that recessed part to affect responsiveness and the change of the intake air negative pressure becomes abnormal and the control becomes difficult since the cross sectional intake direction width is relatively narrow.

A carburetor is equipped with a front surface 35 of an intake air upstream side and a back surface 36 of an intake air downstream side on the cross section of the venturi piston 3 for regulating a throttle of an intake air passage 6 provided in a carburetor body 2. The front surface 35 is made to be a curved surface convex toward the intake air upstream side, and the back surface 36 is made to be a flat shape. The width of the intake air direction in the cross section of the venturi piston 3 is made so that the width of the central part in the cross section is the widest and the width gradually becomes narrow toward the end of the cross sectional direction. The control is facilitated by loosening the change of the intake air negative pressure with the cross sectional shape to reduce the pool of the fuel in the back surface 36 side.