PURPOSE: To produce a desired amt. of process water by sensing a raw water temp., feed pressure of the raw water, the amt. of the raw water, flow rate of concentrated brine, and the amt. of the produced water, and controlling automatically the number of revolution of a high pressure pump basing on the sensed values so as to adjust the feed pressure of the raw water.

CONSTITUTION: Signals for the raw water temp., feed pressure of the raw water, flow rate of the concentrated brine, and for the flow rate of the produced water from a water temp. indicator 8, pressure gauge 10, and flow meters 11, 12 are inputted into an operator 13 in a reverse osmotic membrane separation device comprising a raw water feed piping 5, concd. brine discharging piping 6, and a produced water discharging piping 7, in which a reverse osmotic membrane separator 4 and a high pressure pump 9 are installed, and the feed pressure of the raw water and necessary number of revolution of the high pressure pump 9 for resulting the feed pressure so as to obtain a desired rate of recovery and desired amt. of the produced water are operated basing on the inputted data, and the number of revolution of the high pressure pump 9 is adjusted by a controller 14. By this constitution, necessary amt. of produced water is secured stably in accordance with the change of raw water temp., contamination or consolidation of the reverse osmotic membrane.