To provide a structure integrally injection molded with an intention to improve a water passage with regard to a submerged flat membrane module used in water treatment such as sewage and industrial wastewater.

A submerged flat membrane module 1 fitted with a suction pipe 6 is integrated by injection molding, and a water passage 5 for the water to be treated is constituted on the face and back of the body 2 of the module. The water passage 5 is formed within the region confined by the opposed surfaces of a nonwoven fabric 3 and a filtration membrane 4 and is constituted from marginal water passage grooves 7, water passage openings 9, and communication grooves 10. Each of the water passage openings 9 is formed by confinement by a crosspiece 8, and the crosspiece 8 has a function of preventing warpage.