Abstract: When recovering water by performing a reverse-osmosis-membrane separation treatment on wastewater containing a cationic surfactant such as process wastewater from the electronics industry, high-quality treated water is obtained consistently over the long term while preventing blockage of the reverse-osmosis membrane caused by the cationic surfactant. A reverse-osmosis-membrane separation treatment is performed using a first reverse-osmosis-membrane separation device (3) on wastewater containing a cationic surfactant such as process wastewater from the electronics industry after first adjusting the same to a pH of 3-5, a reverse-osmosis-membrane separation treatment is performed using a second reverse-osmosis-membrane separation device (4) on permeated water from the first reverse-osmosis-membrane separation device (3) after first adjusting the same to a pH of 6.5-10.5, and the permeated water from the second reverse-osmosis-membrane separation device (4) is recovered as treated water.