Disclosed is a membrane module unit in which a plurality of membrane modules is coupled together in parallel by coupling means, the membrane module including a sheet-like filter membrane and holding frame members that hold the filter membrane at two opposed sides of a peripheral edge of the filter membrane, wherein the coupling means clamps the holding frame members at the same sides of the filter membranes together by two opposed clamping members. According to the invention, provided is a membrane module unit in which structure is simple, assembly, disassembly, and maintenance can be easily performed, and a plurality of membrane modules can be reliably coupled together. Provided are also an assembly method, disassembly method, and maintenance method of the same, and a water-treatment apparatus.