Provided are a method of stabilizing a leuco dye storable as a liquid for a long time, a method of reducing nonspecific color development thereof at a time of a color development reaction, and a stable liquid reagent composition using the methods. The inventors of the present invention found that coexistence of the leuco dye with a specific reducing agent resulted in suppression of self color development and its remarkably improved stability in a solution, and that when a color development reaction of the leuco dye with hydrogen peroxide was performed, coexistence of the leuco dye, in a reaction solution, with another dye which had an absorption spectrum not influencing a measurement wavelength of the leuco dye and did not react with hydrogen peroxide suppressed nonspecific color development and lowered a reagent blank value, which was applied to an analytical reagent.