A combined chlorine agent having a low concentration of free chlorine and a high concentration of combined chlorine whereby the combined chlorine concentration of water can be increased when added to water systems at a low concentration of free chlorine. The combined chlorine agent is an aqueous agent which contains an alkali metal hydroxide, sulfamic acid, and an oxidizing agent based on chlorine, wherein the compositional ratio of the oxidizing agent based on chlorine to the sulfamic acid is in the range from 0.45 to 0.6 by Cl/N (mole ratio), the compositional ratio of the oxidizing agent based on chlorine to alkali is in the range from 0.3 to 0.4 by Cl/alkali metal (mole ratio), and the free chlorine concentration in the aqueous agent is 2% by weight or lower of the total chlorine concentration.
