A membrane casing that watertightly houses a membrane element for filtering raw water includes a tubular casing main body, a lid body, and a pressing mechanism. The casing main body houses a membrane element. The lid body fits an opening end of the casing main body and is movable in the axial direction of the casing main body while maintaining the attitude and while securing watertightness with the casing main body. The pressing mechanism and presses the lid body to press and hold the membrane element housed in the casing main body. The membrane casing has a simple configuration capable of watertightly securing using a predetermined pressing force by accommodating variations in size of the membrane element, even if the membrane casing cannot be formed into a cylindrical shape.