1. A document processing apparatus that determines whether or not a document image has watermark information embedded therein by use of character spacing, comprising: an input unit adapted to input a document image; a character-spacing value extraction unit adapted to extract character spacing values in the document image; a calculation unit adapted to calculate fluctuations in the character spacing values; and a determination unit adapted to determine whether or not the document image has watermark information embedded therein by comparing a value of the fluctuations calculated by the calculation unit with a preset threshold value.