PURPOSE: To provide a plastic lens with a concealed mark having excellent water resistance without damaging optical surfaces of the lens, by a method wherein a mark is printed in an aqueous dispersion of a sublimable fluorescent dye on a surface of a plastic article, followed by heating.

CONSTITUTION: A mark is printed in an aqueous dispersion of a sublimable fluorescent dye instead of a dispersion of the dye in an organic solvent, followed by heating. A derivative of 1,8-naphthalenedicarboxyimide of the formula, wherein X is H or lower alkyl, Y is H, lower alkyl, alkoxy, nitro, halogen or OH and Z is H, lower alkyl, alkoxy, nitro, halogen or OH, is used as the sublimable fluorescent dye, preferably, in an amount of 0.1W10wt%. The heating is preferably conducted at 60W120°C in the absence of any gas flow.