A digital watermark embedding processor and digital watermark embedding-processing method set the lowest-common-multiple picture size of the sizes of an unconverted picture and a converted picture when embedding digital watermarks on certain data having a possibility of format conversion and set, for the lowest-common-multiple picture size, the values of the digital watermarks in the converted picture. Based on the set values, digital watermarks for the unconverted picture are set and are embedded in the picture. In this construction, even when a watermark-embedded picture is processed for conversion, it is ensured that accurate detection from the converted picture of watermarks can be performed.