PURPOSE: To permit a preparatory diaphragm in substitution to fulfill the function of a diaphragm facing to an negative-pressure chamber even if said diaphragm is broken, by installing the preparatory diaphragm onto the diaphragm on the opposite side to the fuel chamber of the negative-pressure chamber of a fuel adjustor.

CONSTITUTION: A fuel adjustor 3 is constituted of three chambers of a fuel chamber 16, negative-pressure chamber 18, and an air chamber 20 which are partitioned by diaphragms. The fuel chamber 16 is installed midway into a fuel passage 10, and partitioned from the negative-pressure chamber 18 by the first diaphragm 17, and the negative-pressure chamber 18 is equipped with two diaphragms of the second diaphragm 19a and a preparatory diaphragm 19b on the opposite side. The second diaphragm 19a is made of thin polyethylene, and the preparatory diaphragm 19b is made of rubber.