Provided is a water treatment method using a reverse- osmosis membrane, the method suppressing slime contamination even on a secondary side of the reverse - osmosis membrane, and suppressing oxidative deterioration of the reverse - osmosis membrane. A 5 water treatment method using a reverse- osmosis membrane, in which water to be tr eated is treated using the reverse - osmosis membrane, wherein a nantimicrobial agent is caused to be present in the water to be treated , which contains ammonia, the antimicrobial agent containing a bromine - based oxidizer or a chlorine - based oxidizer , and a sulfamic acid compound. 10
