PURPOSE: To render the coupling of a piston and a piston rod simple and reliable in an apparatus in which a cup-shaped diaphragm is interposed between the piston and a cylinder.

CONSTITUTION: In the apparatus of this invension, a fixed flange 29 fixed to the outer circumference of a piston rod 28 in the manner that it is projected from the outer surface of the piston rod 28 is held tightly between a holding member 31 an a fixing member 24 so as to transmit the reciprocal movement of a piston 11 to the piston rod 28. By employing such an arrangement, it is enabled to prevent loosening of the piston rod 28 from the piston 11, to simplify the assembling operation and to reduce the number of component parts required for constituting the apparatus.