A submerged membrane separator comprising: a filtration membrane arranged on a surface of a flat membrane cartridge; a plurality of the membrane cartridges arranged at predetermined intervals between the filtration membranes of the membrane cartridges opposed to each other; the membrane cartridges are provided in a casing opened at a top thereof; air diffuses for generating upflow in one direction along a membrane surface of the membrane cartridge is provided below the membrane cartridge; and wall members on both sides of a channel of the flow in the one direction formed between the membrane cartridges adjacent to each other, wherein the membrane cartridge is insertable into and removable from the casing in a sideward direction orthogonal to the flow in the one direction and orthogonal to an arrangement direction of the membrane cartridges; the casing for membranes includes a frame body, a pair of front and rear end covers for covering both front and rear ends of the frame body in an arrangement direction of the membrane cartridges, and a pair of left and right side panels (39a, 39b) detachably attached to both left and right sides of the frame body where the frame body includes a bottom frame having a square frame shape, an upper frame having a square frame shape, and a plurality of vertical frames (43a, 43b) coupled between four corners of the frames (41, 42); upper guiding members and lower guiding members have a comb-tooth shape for guiding the membrane cartridge in the sideward direction and are provided in the inside of the casing for membranes; the upper guiding members are provided horizontally in pairs in the upper frame of the frame body and the lower guiding members are provided horizontally in pairs in the bottom frame; a plurality of front and rear guiding grooves are formed in each of the upper and lower guiding members (70, 71) such that the upper end of the membrane cartridge is freely insertable into and removable from the guiding grooves of the upper guiding member from the sideward direction and the lower end of the membrane cartridge is freely insertable into and removable from the guiding grooves of the lower guiding member from the sideward direction, characterized in that the pair of upper guiding members and associated plurality of front and rear grooves are arranged apart from each other at an interval in a sideward direction of the membrane cartridge and the pair of upper guiding members and associated plurality of front and rear grooves are arranged such that they engage the upper edge of the membrane cartridge only at both end portions in a sideward direction of an upper edge of the membrane cartridge when the membrane cartridge is inserted at the final position, and the pair of lower guiding members and associated plurality of front and rear grooves are arranged apart from each other at an interval in a sideward direction of the membrane cartridge and the pair of lower guiding members and associated plurality of front and rear grooves are arranged such that they engage the lower edge of the membrane cartridge only at both end portions in a sideward direction of a lower edge of the membrane cartridge when the membrane cartridge is inserted at the final position. a filtration membrane arranged on a surface of a flat membrane cartridge; a plurality of the membrane cartridges arranged at predetermined intervals between the filtration membranes of the membrane cartridges opposed to each other; the membrane cartridges are provided in a casing opened at a top thereof; air diffuses for generating upflow in one direction along a membrane surface of the membrane cartridge is provided below the membrane cartridge; and wall members on both sides of a channel of the flow in the one direction formed between the membrane cartridges adjacent to each other, wherein the membrane cartridge is insertable into and removable from the casing in a sideward direction orthogonal to the flow in the one direction and orthogonal to an arrangement direction of the membrane cartridges; the casing for membranes includes a frame body, a pair of front and rear end covers for covering both front and rear ends of the frame body in an arrangement direction of the membrane cartridges, and a pair of left and right side panels (39a, 39b) detachably attached to both left and right sides of the frame body where the frame body includes a bottom frame having a square frame shape, an upper frame having a square frame shape, and a plurality of vertical frames (43a, 43b) coupled between four corners of the frames (41, 42); upper guiding members and lower guiding members have a comb-tooth shape for guiding the membrane cartridge in the sideward direction and are provided in the inside of the casing for membranes;