the invention provides an identification data insertion and detection system for digital data which improves the detection ratio when an image is converted into frequency components and electronic watermark data are embedded in a portion of an image having a strong frequency component and besides allows a statistical similarly to be calculated with a higher degree of accuracy . the identification data insertion system for digital data includes a dct calculator for extracting blocks of 8 × 8 pixels from within a current image and performing dct calculation for the blocks , a quantizer for quantizing data outputted from the dct calculator , a quantization table for being referred to when the quantizer performs quantization , an electronic watermark data insertion element for inserting electronic watermark data into data after quantization , and an encoder for encoding data outputted from the electronic watermark data insertion element to produce mpeg data in which the electronic watermark data are inserted and outputting the mpeg data .