Disclosed is a treatment apparatus capable of inhibiting growth of microorganisms and carrying out long-term stable treatment in an activated carbon tower and a reverse osmosis membrane separation device during processes including activated carbon treatment and RO membrane separation treatment of a later step in a system for manufacturing ultrapure water used in an electronic device factory. A method for treating organic-matter-containing water includes: the slime-controlling-agent-adding step of adding a slime-controlling agent to organic-matter-containing water; the activated-carbon-treating step of treating with activated carbon the organic-matter-containing water having undergone the slime-controlling-agent-adding step; and the reverse-osmosis-membrane-separation step of passing the organic-matter-containing water having undergone the activated-carbon-treating step through means for reverse osmosis membrane separation, wherein the slime-controlling agent uses a combined chlorine agent produced from a chlorine-based oxidizer and a sulfamic acid compound.