A diaphragm-clamping arrangement, in which a spring washer having a frusto-conical shape is employed between the outer periphery of the diaphragm and one of the plate members between which the diaphragm is secured. In the free state of the spring washer, the spring washer height is greater than the space between the diaphragm and one plate member so that, when the plate members are joined together, the spring washer is compressed to thereby exert a clamping force on the diaphragm. The diaphragm-clamping point can be provided by either the inner or outer periphery of the spring washer, depending upon whether the frusto-conical shape is concave or convex. By forming an annular recess in the plate member with which the spring is engageable, proper location of the spring washer is facilitated during assembly, and subsequently maintained during clamping.