Provided is a water processing 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 water processing method using a reverse-osmosis membrane, in which water to be processed is processed using the reverse-osmosis membrane, wherein a germicidal agent is caused to be present in the water to be processed, which contains ammonia, the germicidal agent containing a bromine-based oxidizer or a chlorine-based oxidizer, and a sulfamic acid compound.
