The present invention provides a composition for measuring residual chlorine concentration in the sample water, which is an aqueous solution comprising a specific benzidine compound, e.g., N,N'-bis(2-sulfoethyl)-3,3'-dimethylbenzidine disodium salt, and a surfactant, wherein the content of the surfactant is set to be 0.5% by weight or more and to be 0.5 weight-fold or more of the content of the benzidine compound, and the pH is adjusted to be in the acidic region. This composition can prevent the benzidine compound from crystallizing in the aqueous solution under the low temperature condition such as in winter season or the like.