PURPOSE: To eliminate piping running between a carburetor and a drain pump, to dispense with a new drive source for driving the drain pump, and to simplify the mounting structure of a drain pump and reduce the size thereof.

CONSTITUTION: The fuel tank 1 of a gasoline engine E and the float chamber 5 of a carburetor 4 intercommunicate through a fuel feed passage 6, the drain port 7 of the bottom of a float chamber 5 communicates with a fuel tank 1 through a fuel return passage 12, and a drain pump 30 is located in the fuel return passage 12. Only in such a state that the cock handle 29 of a flow passage switch 8 is situated in a drain position B, fuel in the float chamber 5 of the carburetor 4 is returned to the fuel tank 1. The drain pump comprises a diaphragm drain pump 30 installed integrally with the carburetor, and the pulsation part 40 of an engine E is utilized as the drive source of the diaphragm 30.