To provide a flat membrane module which can maintain longer filtration duration time than the conventional art by eliminating the deviation of a crossflow flow rate produced on the flat membrane surface, and can suppress the cleaning frequency of a chemical washing low, and a water treatment apparatus using the same.

The flat membrane module includes an aggregate of a plurality of flat membrane elements 1 disposed at regular intervals in a horizontal direction, an air diffuser 2 arranged below the flat membrane element aggregate, and a chassis 3 surrounding them from four directions by blocking them from the outside from the horizontal direction. The flat membrane module has a space 6 opened at the upper end between the both side surfaces of the flat membrane element aggregate and the inner wall of the chassis. The side surface of a gap 7 between the flat membrane elements communicates with the space 6.