PURPOSE: To prevent unnecessary fuel discharge by making it possible to lock a diaphragm by the action of the second diaphragm at stoppage of an engine or in high temperature condition thereof in the case wherein a pump action is obtained by changing the diaphragm by means of an intake negative pressure.

CONSTITUTION: An acceleration pump casing 17 is formed in one body on one side of a casing 10 in which the float chamber 9 of a carburettor is formed and a fuel chamber 12, a negative pressure chamber 18 and the second negative pressure chamber 19 are formed in series by means of both casings 10, 17. The fuel chamber 12 and the negative pressure chamber 18 are separated from each other by a diaphragm 11 and two negative pressure chambers 18, 19 are separated from each other by a partition 21 and the second negative pressure chamber 19 is separated from the air side by the second diaphragm 23. One end of a rod 22 is fixed to the diaphragm 11 and the other end is locked in engaged form to on engaging-lock member 30 provided in the second diaphragm 23. The intake negative pressure to be led to the second negative pressure chamber 19 is controlled in its charge and discharge by a control means 25 to make it possible to switch the second diaphragm 23 between the position to allow a pump operation and its stoppage position.