The present invention provides a membrane filtration system (1) including: a filtering membrane portion (3) for removing impurities in feed water; a drain line (16) for draining a part of concentrate from the filtering membrane portion (3) to an exterior of a system; a concentrate return line (17) for returning a remainder of the concentrate from the filtering membrane portion (3) to an upstream side of the filtering membrane portion (3); a return flow rate adjusting portion (28) for concentrate provided in the concentrate return line (17); and a control portion (30) controlling the return flow rate adjusting portion (28) based on a drain flow rate of the concentrate or a flow rate of product water from the filtering membrane portion (3). With this construction, it is possible to suppress wasteful power consumption in a feed pump (7) for feeding water to the filtering membrane portion (3) and to prevent clogging in the filtering membrane in the filtering membrane portion (3).