To provide a substance contained in an aqueous liquid labeled with a dye useful for detection.

A method of labeling a component in the aqueous liquid includes a step (a) of adding a luminescent dye selected out of a group composed of a sulfoindolenin group agent and a naphthosulfoindolenin-group polymethylene dye containing at least one sulfonic group or sulfonate group attached to one heterocyclic nucleus or more than one rings and having reactivity to the component to the aqueous liquid containing the component, a step (b) of causing the dye and component to act upon another so that the dye may label the component, and a step (c) of detecting the labeled component by an optical method.