To obtain the subject ink capable of readily visually observing printed result and making the printed result invisible or slightly visible by subjecting the printed material to decoloring treatment.

A material which is invisible or slightly visible when dissolved in cyanine dye, is used as a fluorescent material used for fluorescent ink. The fluorescent ink is mixed with a decolorizer such as cyanine dye which decolors when a specific wavelength is irradiated to the fluorescent ink. In this case, a cyanine dye exhibiting absorption at a wavelength longer than that excited with cyanine dye of the fluorescent material is used as the decolorizer.