An object of the present technology is to provide a chlorine-concentration-measuring composition that can reduce staining by reagents, a measurement method using the chlorine-concentration-measuring composition and a method for reducing staining by chlorine-concentration-measuring composition using an aromatic sulfonic acid-based polymer or the salt thereof.Provided are a chlorine-concentration-measuring composition comprising an aromatic sulfonic acid-based polymer or the salt thereof; a chlorine-concentration-measuring composition comprising component (a) a color reagent for detection of residual chlorine and component (b) an aromatic sulfonic acid-based polymer or the salt thereof; a chlorine-concentration-measuring method comprising using the composition; and a method for reducing or preventing staining by using the chlorine-concentration-measuring composition comprising an aromatic sulfonic acid-based polymer or the salt thereof.