A flat-membrane element is rendered likely to vibrate by air bubbling from an aeration tube. The element has a sheet-shaped filtering membrane that has oppositely positioned portions placed with a space between them being provided as a flowing path for a treated liquid, a supporter that is placed between the oppositely positioned portions of the filtering membrane to secure the space as a flowing path for a treated liquid, and a periphery-sealing portion that seals the peripheral edges of the oppositely positioned portions of the filtering membrane with a treated-liquid outlet being provided. The filtering membrane and supporter are formed of flexible material and the periphery-sealing portion has flexibility, thereby rendering the entire element flexible. The flat-membrane element has a total thickness of 6 to 1 mm.
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