Line spacing values in a document image are extracted and a variance is calculated for fluctuations in the line spacing values. When the calculated variance is higher than a preset threshold value, the document image is determined as having watermark information embedded therein. Such use of the variance in the line spacing values enables high-speed determination of the presence or absence of line-spacing watermark information. At this time, it is possible to speed up the determination by using only some line spacing values in the document, instead of using all line spacing values.