To produce a diaphragm excellent in durability becaused of its high mechanical strength and fatigue strength, in which particles caused by corrosion is hard to generate because of its excellent corrosion resistance, furthermore, gases high in decomposability is hard to generate since the surface shows excellent noncatatytic properties and, moreover, out gases are hardly generated.

This diaphragm is the one using a material obtd. by subjecting a working hardening type and aging hardening type Co-Ni base alloy e.g. a Co-Ni base alloy having a compsn. contg, by weight, 30 to 40% Co, 27 to 36% Ni, 12 to 26% Cr, 8 to 13% Mo, one or ≥ two kinds among Mn, Ti, Al and Fe respectively by 0.1 to 3%, 0.5 to 3% Nb and 0.005 to 0.05% misch metal with inevitable impurities to cold rolling at 20 to 90% final working degree.