To provide a diaphragm for an accumulator capable of enhancing the durability by generating an elastically deformed condition in good performance.

The diaphragm is to partition the hollow chamber in the accumulator into a gas chamber and a liquid chamber and is equipped with a resin layer 1 having an approximately hollow hemispherical shape, an outer elastic material layer 2 and an inner elastic material layer 3 formed in consolidation to the two surfaces of the resin layer 1, a poppet 4 made of metal or resin installed in the center of the peripheral surface of the outer elastic material layer 2, and 16 ring-shaped ribs 5a-5p formed on the inside surface of the inner elastic material layer 2 in such a way as extending in the circumferential direction around the center of the inner elastic material layer 2. The ring-shaped ribs 5a-5p are arranged so that their stiffness heightens in steps at four-piece intervals as going outward from the center.