An electronic watermark data insertion apparatus comprises an insertion information memory (39) for preliminarily storing insertion information for designating electronic watermark data to be inserted block by block and an electronic watermark data insertion unit (33) for inserting, on the basis of the insertion information, the designated electronic watermark data from an electronic watermark data table (37) in a quantized image block by block. An electronic watermark data detection apparatus comprises an electronic watermark date extracting unit (45) for extracting the image data having a biased frequency region projecting in dependency on the inserted electronic watermark data block by block and an electronic watermark data detection unit (50) for calculating a statistical similarity between the extracted data and the electronic watermark data stored in an electronic watermark data table (48). An electronic watermark data accumulator (51) accumulates the statistical similarity for a predetermined time interval to produce an accumulated addition result which is reset after the elapse of the predetermined time interval or when the accumulated addition result exceeds the a predetermined threshold value. A determining unit (52) determines that the electronic watermark data is detected when the accumulated addition result exceeds the predetermined threshold value.