The present invention provides a water treatment agent that exhibits the effect of hypobromous acid when added to a water system. The present invention is a water treatment agent containing the following components (a) to (c) and having a pH of 10 or higher, and/or a water treatment agent containing an alkali agent, a chloramine compound, and a bromide salt, wherein the component ( a)～component (c) is (a) chloramine compound, (b) bromide salt, (c) the carboxyl group content rate in the polymer is 0.8 g-COOH/g-polymer or less carboxyl polymer 1% by mass ~18% by mass, a water treatment agent, the total chlorine detection rate after manufacture is 95% or more, and the free chlorine content in the total chlorine concentration is 0.05% (as Cl 2 ) or less. The molar ratio of the chloramine compound and the bromide salt is preferably 1:0.1 to 1.0.本發明提供一種於添加到水系中時次溴酸的效果良好地發揮的水處理劑。本發明是一種含有以下的成分（a）～成分（c）且pH為10以上的水處理劑、及/或含有鹼劑、氯胺化合物及溴化物鹽的水處理劑，其中所述成分（a）～成分（c）為（a）氯胺化合物、（b）溴化物鹽、（c）聚合物中的羧基含有率為0.8 g-COOH/g-聚合物以下的羧基聚合物1質量%～18質量%，一種水處理劑，製造後的總氯檢測率為95%以上且總氯濃度中的游離氯含有率為0.05%（as Cl2）以下。所述氯胺化合物與溴化物鹽的莫耳比較佳為1：0.1～1.0。
