A variable venturi carburetor, which is part of an intake system of an internal combustion engine, includes structural features which allow an air intake path of the carburetor to be shortened, and which allow the size of air chambers of the carburetor to be reduced. A piston valve is composed of a plate-shaped valve and a tubular portion. The plate-shaped valve is guided by a pair of grooves provide in sidewalls of the intake path. The tubular portion is attached to one side of the plate-shaped valve near a center of the side of the plate-shaped valve. The tubular portion has a rectangular or square cross-section. Each side of the rectangular cross section is shorter than the internal diameter of the intake path. A flange is attached to an upper end of the piston valve. The flange is of a reduced diameter and facilitates connection of the piston valve to a diaphragm. Since the diameter of the flange is also less than the diameter of the intake path, a vacuum chamber and an atmosphere chamber of the carburetor can be reduced in size.