To perform mounting of a rod-form piston member and provide a surface area having substantially no protrusion part making immediate contact with fluid fed by a pump and/or other given material.

A flexible diaphragm 10 comprises a first body part 12 providing a first grade of flexibility; and a second body part 14 extending in a manner to surround the periphery of the first body part and providing a second grade of flexibility higher than the first grade of flexibility. A seal member 16 seals a flexible diaphragm extending in a manner to surround the periphery of the second body part 14 and situated in an operation position in a pump housing part. A flexible plastic layer 18 is adhered to parts making direct contact with a material to be fed by a pump or the surfaces 20, 22, and 24 of the member. An insert member 40 is embedded in the first body part and connected to a rod-form piston member reciprocatively disposed in a pump.