Raw water filtration treatment system, and method for cleaning filtration device

The present invention is equipped with: a raw-water supply line L 10 for supplying raw water 11; a filtration device 12 for filtering impurities contained in the raw water 11, and provided along the raw-water supply line L 10 ; a separation device 23 provided on the downstream side of the filtration device 12 and equipped with a separation membrane 23a for separating the filtered raw water 11A into permeated water 21 and concentrated water 22 in which fine particles or solutes such as salt are concentrated; an organic-matter monitoring device 25A for monitoring the amount of organic matter in the raw water 11 and provided to the front and/or the rear of the filtration device 12; and a control device 31 for controlling so as to use the permeated water 21 for backwashing the filtration device 12, when determined that the amount of organic matter in the raw water 11 exceeds a reference value, on the basis of the monitoring results from the organic-matter monitoring device 25A.