The present invention serves to facilitate atomization of fuel in a diaphragm type carburetor for all-purpose two-cycle engines. A check valve used to prevent back flow is installed in the fuel passage leading from a constant-fuel chamber to a main nozzle, and an air bleed passage is connected at a position on the downstream side of the check valve. The main nozzle is constructed by forming nozzle openings which face downstream with respect to the engine intake air flow. The nozzle openings are located in a tubular member which cuts across a central axial line of the venturi so that the tubular member bridges the neck of the venturi.