Provided are a coloration agent for carbonyl sulfide which contains a metal oxide, and at least one metal component selected from among a cerium component, a silver component, a copper component, a nickel component, and an iron component, the metal component being supported on the metal oxide, or which contains at least one metal oxide selected from among a cerium oxide, a silver oxide, a copper oxide, a nickel oxide, and an iron oxide; carbonyl sulfide detection means including the coloration agent for carbonyl sulfide; and a fuel cell system including the detection means. The coloration agent for carbonyl sulfide has the ability to develop a color in the presence of carbonyl sulfide even at ambient temperature, exhibits excellent performance in detecting carbonyl sulfide contained in a hydrocarbon fuel, and enables the service life of a desulfurizing agent to be determined through visual observation.