To obtain a color-developing ink composition for offset printing, containing a leuco dye, hardly causing reduction of the density of a developed color with time, and hardly causing bleeding of a printed product and staining of a penpoint even if the writing by a writing utensil filled with the ink containing a developer is carried out.

The color-developing ink composition for the offset printing comprises the leuco dye, a binder resin, a solvent and an auxiliary agent for dissolving the dye. The reaction with the developer is smoothly carried out to make the color development good, and the reduction of the density of the developed color with time is prevented because the leuco dye is dissolved by using the solvent and the auxiliary agent for dissolving the dye. The bleeding of the printed product and the staining of the penpoint are not caused even if being written by the writing utensil filled with the ink containing the developer because the leuco dye is present by being dissolved in the coated film of the printed product.