To develop forgery-proof paper which easily clarifies forgery when forgery is done with an organic solvent (e.g. an alcohol) and a chemical (e.g. an ink eraser), and excels in storage stability such as whiteness, heat resistance, rub resistance and water resistance, and a forgery-proof aqueous ink composition suitable for obtaining the same.

The color developing ink composition comprises a leuco dye which is normally a colorless or light-colored color-developing compound, a developer compound which causes color development of the leuco dye on heating, and a sulfuric acid salt (A) of a compound represented by formula (a). In the ink composition, the leuco dye is 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide; and the developer compound is a compound (B) having a melting point of ≥240°C.