An embodiment of the present invention includes: a spiral type pressure vessel 15 in which a plurality of reverse osmosis membrane apparatuses 13-1 to 13-10 having spiral reverse osmosis membranes is connected through a permeated water pipe 14, and is housed in a connected state; a raw water supplying line that supplies raw water 11 into the pressure vessel 15; a concentrated water discharging line through which concentrated water 16 concentrated is discharged; a plug 17 that blocks the permeated water pipe 14 at the center of the reverse osmosis membrane apparatuses 13-1 to 13-10; a front-side permeated water line and a front-side permeated water line through which front-side permeated water 12-1 and rear-side permeated water 12-2 are discharged to the exterior, respectively, which are separated fore and aft, respectively, of the permeated water pipe 14 blocked by the plug 17; a pressure regulating valve 20 that is mounted in the raw water supplying line and regulates the supply pressure of the raw water 11; and a flow regulating valve 22 that is mounted in the front-side permeated water line and regulates the pressure of the front-side permeated water 12-1.