The present invention is directed to the detection of what part of an image alteration was added to by embedding an electronic watermark in the image. More particularly, embedding part 30 renders as pairs two each of DCT blocks of luminance component Y of an image, randomly selects, by using a random number, mutually corresponding DCT coefficients from each DCT block contained in the pairs, and manipulates so that the relationship among these DCT coefficients represent data according to a certain rule, and then embeds the data. If alteration is added to this image, the DCT coefficients contained in the pairs in the altered part do not observe the above rule and represent different values from the original data. Extraction part 40 extracts data from the image, and makes decision by majority to estimate the initially embedded data to detect and display the pairs from which different data form the estimated data was extracted as those to which alteration was added.