PROBLEM TO BE SOLVED: To provide an electronic watermark embedding method and device that precludes the possibility of affecting the image quality of an image by an embedded electronic watermark, makes counterfeit difficult and can be applicable to the image even in the case of compression/expansion processing of the image. SOLUTION: A data conversion means 11 converts image data. Furthermore, a random number sequence generating means 14 generates a pseudo random number by using a random number seed generated by a random number seed generating means 13, and outputs generated pseudo random number as a watermark series to a watermark data embedding means 12. The watermark data embedding means 12 sequentially embeds data of the watermark series to the converted image data to obtain watermarked data sequentially. In this case, the data of the watermark series to be embedded to obtain succeeding watermarked data are revised on the basis of the result of comparison of the obtained watermarked data and a preset threshold value.