A membrane separation device comprising: a membrane module having a plurality of membrane elements, each including a membrane support body in a form of a flat panel or sheet and a separation membrane attached to a front surface and a rear surface of the support body, the plurality of membrane elements being arranged in a longitudinal position such that the separation membranes thereof face one another with a fixed distance provided therebetween; a narrowing member located outside of an outermost membrane element which is disposed at an outermost position along the arrangement direction, the narrowing member being disposed at least in a vicinity of a bonding portion of the membrane support body and the separation membrane of the outermost membrane element such that the narrowing member is separated from the outermost membrane element by a predetermined distance (t) smaller than the fixed distance; a diffuser air supply pipe is provided under the membrane module such that the diffusion air supplied by the diffuser air supply pipe causes an upward flow of the water to be treated between a plurality of membrane elements which are in a longitudinal position and laterally arranged in a horizontal direction in each of the membrane module; and a pair of cover members disposed on respective outer side of the outermost membrane elements in the arrangement direction so as to have a certain distance from the corresponding outermost membrane elements such that a flow path for the water to be treated is formed, wherein each of the cover members includes a narrowing member formed on a surface thereof which faces the outermost membrane element disposed at the outermost position along the arrangement direction, the narrowing member is formed at an upper part of the vicinity of the bonding portion, characterized in that the narrowing member includes a protrusion which is formed on a surface of the cover member facing the bonding portion of the outermost membrane element disposed at the outermost position along the arrangement direction so as to have a thickness greater than that of a middle portion of the cover member, the protrusion and the middle portion are connected to each other by a sloped surface or a curved surface. a membrane module having a plurality of membrane elements, each including a membrane support body in a form of a flat panel or sheet and a separation membrane attached to a front surface and a rear surface of the support body, the plurality of membrane elements being arranged in a longitudinal position such that the separation membranes thereof face one another with a fixed distance provided therebetween; a narrowing member located outside of an outermost membrane element which is disposed at an outermost position along the arrangement direction, the narrowing member being disposed at least in a vicinity of a bonding portion of the membrane support body and the separation membrane of the outermost membrane element such that the narrowing member is separated from the outermost membrane element by a predetermined distance (t) smaller than the fixed distance; a diffuser air supply pipe is provided under the membrane module such that the diffusion air supplied by the diffuser air supply pipe causes an upward flow of the water to be treated between a plurality of membrane elements which are in a longitudinal position and laterally arranged in a horizontal direction in each of the membrane module; and a pair of cover members disposed on respective outer side of the outermost membrane elements in the arrangement direction so as to have a certain distance from the corresponding outermost membrane elements such that a flow path for the water to be treated is formed, wherein each of the cover members includes a narrowing member formed on a surface thereof which faces the outermost membrane element disposed at the outermost position along the arrangement direction, the narrowing member is formed at an upper part of the vicinity of the bonding portion, characterized in that the narrowing member includes a protrusion which is formed on a surface of the cover member facing the bonding portion of the outermost membrane element disposed at the outermost position along the arrangement direction so as to have a thickness greater than that of a middle portion of the cover member, the protrusion and the middle portion are connected to each other by a sloped surface or a curved surface.