To provide a digital data processing method for inserting digitized information into digital data, and more specifically, provide a digital watermarking technique which inserts various information of a copyright, an identification code, or the like into digital image data and digital speech data.

In a digital data processing method having a means for inputting digital data and a means for inserting digitized information into the digital data, a digital data stream is assorted in accordance with a characteristic of the digital data stream, and a means for inserting optimal information is selected in correspondence to each assorted digital data stream, whereby influences imparted to a man's audio-visual sense by an image or speech expressed by the digital data into which the information is inserted are prevented.