The fuel supply system includes an air pipe which is connected between a fuel oil supply pipe and an inlet side of a forced air passage and is provided at a middle portion thereof with an elevation positioned above any portion of the fuel supply pipe between the air pipe and a pot of the oil burner. The fuel supply system may include an outflow prevention mechanism wherein the elevation formed at the air pipe is above the elevation of the fuel supply pipe and a return pipe, and the return pipe has an elevation positioned between the elevation of the air pipe and that of the fuel supply pipe and is connected between the oil reservoir and the fuel supply pipe.