1. A non-transitory computer-readable medium storing a digital watermarking program for enabling a computer to embed digital watermark information in text data to generate a digitally watermarked image, the program causing the computer to execute a method comprising: detecting local line thickness at each pixel on lines composing a character from embedding target data with respect to each of a predetermined set of the directions; determining character line thickness for said character with respect to each of said predetermined set of the directions from said local line thickness as features of said character; and embedding digital watermark information in said character by using a ratio of character line thicknesses for said character in at least two of said predetermined set of directions.