Abstract: The rotary flat membrane disk is constructed in which circular membranes are stretched at both sides of a disk. The disk and the membrane are bonded to each other over the entire periphery by a peripheral bonding part, and they are also bonded to each other by a plurality of partial bonding parts arranged inside the peripheral bonding part. The partial bonding parts are formed like dots of 10 mm in diameter, and are arranged at every 30 degrees on circles of 500 mm and 600 mm in diameter.