Apparatus for superimposing a digital watermark in a video signal and apparatus for detecting the same

In a digital-watermark superposing apparatus and a digital-watermark detecting apparatus, a plurality of DCT coefficients for respective frequency components are generated by subjecting video data to a two-dimensional DCT transform for each pixel block consisting of a predetermined number of pixels, and the value of at least one of the DCT coefficients is quantized in accordance with a quantization rule corresponding to the information so as to obtain a quantized DCT coefficient. A two-dimensional inverse DCT transform is performed with respect to the DCT coefficients in which the at least one DCT coefficient thereof is replaced with the quantized DCT coefficient, so as to obtain digital-watermark superposed video data in which the digital watermark corresponding to the information has been superposed on the video data. At this juncture, the digital-watermark superposed video data is selected and outputted concerning a section in which a video pattern due to the video data is not flatter than a predetermined video pattern, while, instead of the digital-watermark superposed video data, the video data is selected and outputted as it is concerning a section in which the video pattern due to the video data is flatter than the predetermined video pattern.