A color conversion module 42 carries out color conversion of original
color image data Grgb from the RGB color
system into the CMYK color
system to obtain color-converted original
color image data Gcmyk (step S104). A DCT module 44 applies DCT (
discrete cosine transform) over the whole color-converted original
color image data Gcmyk to generate DCT coefficients Dcmyk (step S106). An embedding module 46 embeds the
watermark information s into the components C, M, Y, and K of the DCT coefficients Dcmyk (step S108). An IDCT module 48 applies IDCT (inverse
discrete cosine transform) onto DCT coefficients D'cmyk with the
watermark information s embedded therein to generate embedding-processed color image data G'cmyk (step S110). The color conversion module 42 carries out color conversion of the embedding-processed color image data G'cmyk from the CMYK color
system into the RGB color system to obtain embedding-processed color image data G'rgb (step S112). This arrangement does not require any correction of the position or the shape of image blocks in the process of extracting the embedded
watermark information.