On embedding a watermark in a stream of digital images, a watermark strength is automatically varied in accordance with an amount of the digital images by detecting the amount of the digital images by a detection portion. The amount of the digital images may be measured at every unit time, for example, a unit of block, macroblock, slice, picture. Thus, detectability of the watermark is improved by adjusting the watermark strength on the basis of the amount of the digital images.