A watermark information detecting method comprising: image inputting step for reading a printed document with confidential information embedded as an input image, by preparing plural dot patterns with a direction of wave and/or wavelength changed according to an arrangement of dots, assigning each of at least two different symbols to a different one of the plural dot patterns and arranging the plural dot patterns combined with each other; a filtering step for obtaining a filter type matrix and a filter output value matrix by performing filtering of the input image after preparing detection filter for each of the plural dot patterns having the same wave direction and wavelength as said dot pattern and having a type indicating said dot pattern in order to detect the plural dot patterns from the input image, the detection filters being Gabor filters and the filtering being performed by, for each pixel of the input image, calculating filter output values of the detection filters, comparing the filter output values to determine a maximum filter output value, setting the maximum filter output value at said pixel in the filter output value matrix, and setting the type of the detection filter yielding the maximum filter output value at said pixel in the filter type matrix; a position searching step for determining the position of the dot patterns in order for the sum of the output values of the detection filter corresponding to a grid point of a position searching template to be maximum, while moving the position searching template in each area divided in a predetermined size with regard to the filter output value matrix; a symbol determining step for obtaining a symbol matrix by determining the symbol of the dot patterns embedded in a location determined in the position searching step from the type of the detection filter in the filter type matrix, corresponding to the location; a border determining step for determining a border of an area, wherein the border of the area is determined by the dot patterns embedded based on the predetermined dot patterns embedded in the printed document in advance; and an information decoding step for decoding the confidential information embedded in the printed document based on the dot patterns embedded inside the border. image inputting step for reading a printed document with confidential information embedded as an input image, by preparing plural dot patterns with a direction of wave and/or wavelength changed according to an arrangement of dots, assigning each of at least two different symbols to a different one of the plural dot patterns and arranging the plural dot patterns combined with each other; a filtering step for obtaining a filter type matrix and a filter output value matrix by performing filtering of the input image after preparing detection filter for each of the plural dot patterns having the same wave direction and wavelength as said dot pattern and having a type indicating said dot pattern in order to detect the plural dot patterns from the input image, the detection filters being Gabor filters and the filtering being performed by, for each pixel of the input image, calculating filter output values of the detection filters, comparing the filter output values to determine a maximum filter output value, setting the maximum filter output value at said pixel in the filter output value matrix, and setting the type of the detection filter yielding the maximum filter output value at said pixel in the filter type matrix; a position searching step for determining the position of the dot patterns in order for the sum of the output values of the detection filter corresponding to a grid point of a position searching template to be maximum, while moving the position searching template in each area divided in a predetermined size with regard to the filter output value matrix; a symbol determining step for obtaining a symbol matrix by determining the symbol of the dot patterns embedded in a location determined in the position searching step from the type of the detection filter in the filter type matrix, corresponding to the location; a border determining step for determining a border of an area, wherein the border of the area is determined by the dot patterns embedded based on the predetermined dot patterns embedded in the printed document in advance; and an information decoding step for decoding the confidential information embedded in the printed document based on the dot patterns embedded inside the border.