To provide a water treatment device capable of preventing the reduction of a permeated water amount at the time of the water permeation operation and performing a stable long run even if raw water containing turbid components is permeated by selecting proper operating conditions and suitable membranes to regularly or irregularly perform back-pressure washing from a permeation water side in a water treatment device using a reverse osmosis membrane module or nano-filtering membrane module and a method of operating it.

The water treatment device and the method of operating it in the method for operating the water treatment device in which permeated water and concentrated water are separated by supplying the raw water to the reverse osmosis membrane module and nano-filtering membrane module, for the surface roughness of the membrane used in a membrane element of the reverse osmosis membrane module or nano-filtering membrane module when unused, a membrane of difference between the highest point and lowest point in a straight line of arbitrary length of 1 μm on an even polymerization surface is 0.4 μm or less is used, and the membrane is regularly or irregularly back-pressure washed from the water permeation side.