A decolorization treatment method in which not only can a liquid that has been obtained by treating raw water that is stock-raising treatment water in a purifier tank be decolorized swiftly with simple equipment, but moreover the treatment water can be purified to an extent that reuse as a washing liquid or the like is possible. The method of decolorizing stock-raising treatment water comprises adding ferric polysulfate and a nonionic or anionic organic coagulant to a filtrate that has been obtained by treating raw water that is stock-raising treatment water in a purifier tank, and agitating the obtained product to bring about reaction and thus separate into solid and liquid components, and removing the solid component. Preferably, a two-stage reaction is carried out in which the ferric polysulfate is added and agitation is carried out to bring about reaction, and then the nonionic or anionic organic coagulant is added thereafter.