A flat membrane element adapted to be easily vibrated by air bubbling from an air diffusion pipe. A flat membrane element provided with: sheet-like filtration membranes arranged face-to-face so as to be separated from each other to provide a space which serves as the channel for a treated liquid; support elements arranged between the filtration membranes, which are arranged face-to-face, to secure the space which serves as the channel for a treated liquid; and an outer-periphery sealing section for sealing the outer peripheral edge of the filtration membranes, which are arranged face-to-face, without sealing the opening for extracting a treated liquid. The filtration membranes and the support elements consist of flexible materials, the outer-periphery sealing section also consists of a flexible material, the entire flat membrane element is configured to be flexible, and the overall thickness of the flat membrane element is set to be in the range from 6 mm to 1 mm.
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