PROBLEM TO BE SOLVED : To provide a filtration device, wherein, if the filtration device has a multi-stage filtration configuration in which three or more filtration means are connected in series, without midway storage in an intermediate tank or the like, the control in passing of water to be treated becomes easier. SOLUTION : A filtration device has : three or more RO units that allow treated water or permeated water to permeate through a filtration membrane without being stored on the way; and three or more pumps that are provided at the front stage of each of the three or more RO units, and send water to the RO units excluding the RO unit of the final stage so that the pressure of permeated water is within a predetermined range, while sending the water to the RO unit of the final stage so that the flow rate of permeated water is within a predetermined range. SELECTED DRAWING : Figure 2 COPYRIGHT : (C)2019, JPO&INPIT