1. A method of embedding a digital watermark in a master image, said embedding method comprising the steps of: extracting blocks of a predetermined size from said master image; processing image data corresponding to each block by orthogonal transform; comparing orthogonal transformed coefficients between at least two blocks having a predetermined relationship with each other and making the coefficients satisfy a preset order of magnitude according to bit information specified as the digital watermark; quantizing the coefficients obtained by the orthogonal transform with a quantization table and using the quantized coefficients to embed the bit information, wherein embedding the bit information is carried out when, are not all equal to zero; and processing each block with the embedded bit information by inverse orthogonal transform, so as to output a resulting image with the digital watermark embedded therein; introducing a logic function that is true when a difference between the orthogonal transformed coefficients of the at least two blocks having the predetermined relationship is in a preset range; and modifying a procedure adopted to embed the bit information, based on the true and false state of the logic function.