A membrane container 6 has a casing 10 including a fluid inlet 14 and a fluid outlet 16 and a membrane container body 9 including a plurality of flow paths 11 which is arranged along the flow direction of the treated fluid and in parallel to one another. Each of the plurality of flow paths 11 includes a most upstream portion 11A which is connected to the fluid inlet 14, and a most downstream portion 11C which is connected to the fluid outlet 16. Return portions 15 and 17 configured to reverse the flow direction of the treated fluid are provided between the most upstream portion 11A and the most downstream portion 11C. After passing through the fluid inlet 14, the treated fluid flows from the most upstream portion 11A down to the most downstream portion 11C via the return portions 15 and the upper return portion 17.