PURPOSE: To provide a plane membrane structure which can prevent surely a membrane from being broken by the weight of transmitted liquid deposited in the membrane when a membrane element is lifted from a raw liquid tank or the pressure of backwashing in the plane membrane structure using the tank immersion type membrane element.

CONSTITUTION: A transmitted liquid passage material 2 is placed between two plane membranes 1, 1, and the two membranes 1, 1 or each membrane 1 and the passage material 2 are bonded partially 3 with an adhesive at specified intervals. The interval is usually arranged to be 5-100mm.