A membrane module that is immersible in a liquid to be treated to perform filtration, comprising: - a pair of water collecting cases (35a, 35b) and at least one membrane element provided between the pair of water collecting cases (35a, 35b), wherein - water collecting spaces in which permeate having permeated through the membrane element is collected are formed in the water collecting cases (35a, 35b), - the water collecting cases (35a, 35b) can be coupled via coupling portions (87; 110; 141; 145; 151) to water collecting cases (35a, 35b) of another membrane module adjacent to the membrane module in a predetermined arrangement direction, - each of the water collecting cases (35a, 35b) have coupling surfaces (49, 50) at opposite ends thereof for coupling to the water collecting cases (35a, 35b) of the another membrane module, - the coupling portions (87; 110; 141; 145; 151) are provided on the coupling surfaces (49, 50) of the water collecting cases (35a, 35b), - each of the water collecting cases (35a, 35b) have coupling portions (87; 110; 141; 145; 151) with flow holes (99; 117), a fitting hole (89; 112) at one end and a fitting projection (88; 111) at the other end, wherein the fitting projections (88; 111) can be detachably fit into the fitting holes (89; 112) of the another membrane module in the predetermined arrangement direction, wherein - the water collecting spaces communicate with water collecting spaces of the water collecting cases (35a, 35b) of the other membrane module via the flow holes (99; 117) of the coupling portions (87; 110; 141; 145; 151), characterized in that at least one of the fitting projections (88; 111) and the fitting holes (89; 112) are slidable within a predetermined range in a direction orthogonal to the predetermined arrangement direction via a seal member (103; 131; 142) for securing water tightness with the communicating water collecting spaces. - a pair of water collecting cases (35a, 35b) and at least one membrane element provided between the pair of water collecting cases (35a, 35b), wherein - water collecting spaces in which permeate having permeated through the membrane element is collected are formed in the water collecting cases (35a, 35b), - the water collecting cases (35a, 35b) can be coupled via coupling portions (87; 110; 141; 145; 151) to water collecting cases (35a, 35b) of another membrane module adjacent to the membrane module in a predetermined arrangement direction, - each of the water collecting cases (35a, 35b) have coupling surfaces (49, 50) at opposite ends thereof for coupling to the water collecting cases (35a, 35b) of the another membrane module, - the coupling portions (87; 110; 141; 145; 151) are provided on the coupling surfaces (49, 50) of the water collecting cases (35a, 35b), - each of the water collecting cases (35a, 35b) have coupling portions (87; 110; 141; 145; 151) with flow holes (99; 117), a fitting hole (89; 112) at one end and a fitting projection (88; 111) at the other end, wherein the fitting projections (88; 111) can be detachably fit into the fitting holes (89; 112) of the another membrane module in the predetermined arrangement direction, wherein - the water collecting spaces communicate with water collecting spaces of the water collecting cases (35a, 35b) of the other membrane module via the flow holes (99; 117) of the coupling portions (87; 110; 141; 145; 151),