There is provided a watermark information detecting method capable of detecting confidential information accurately from a document including confidential information. In this method, a filtering process is performed on the whole surface of an input image (S310), and a position of signal is obtained by using a signal position searching template in order for the sum of filter output value to be maximum (S320). Then a signal border is determined (S340). Even when the image is expanded or contracted due to displacement of paper, etc., the signal position can be correctly detected and confidential information can be correctly detected from a document including confidential information.