PURPOSE: To improve a sealing property by providing a diaphragm having a sagging which permits the reciprocation of a piston in between the outer periphery of the piston and the inner wall of a pump chamber.

CONSTITUTION: A piston 7 is housed in a pump chamber 6 reciprocatingly movably right and left. The inside part of a diaphragm (d) is held by and fixed to the part between a piston receiver 7b and a piston body 7a. And, the outside part of the diaphragm (d) is placed in the gap (t) between the outer periphery of the piston body 7a and the inner wall of the pump chamber, with a sagging having a length which permits the reciprocation of the piston being provided, thereby, completely sealing the 6a and 6b of the pump chamber.