PURPOSE: To maintain a pressure drop generated in a bypass during idling so as to avoid mixture insufficiency due to the reduction of pump capacity by closing a bypass duct connecting a fuel supply portion and a pressure portion with a valve, and controlling the valve with the pressure in the delivery portion.

CONSTITUTION: A diaphragm fuel pump 5 comprises a drive chamber 8 receiving the inner pressure of a crankcase 9 of an internal combustion engine 1, and a pump chamber 7 partitioned form the drive chamber 8 by a diaphragm 6. The pump chamber 7 is connected to a fuel duct via a first check valve 4 and a suction portion 40. And the pump chamber 7 is connected to a pressure controller 15 of a diaphragm carburetor 29 via a second check valve 10 and a pressure portion 50. The pressure portion 50 is connected to a suction portion 40 via a bypass duct 70 with a throttle 71. In such a structure, the bypass duct 70 is closed by a valve 72. A valve member 61 of the valve 72 is controlled by the pressure in the supply portion 50.