A secure electronic watermark inserter. A picture analyzer 3 analyzes an input picture 101, and decides the insertion strength of the electronic water marking to be inserted, from one pixel of the input picture 101 to another, to output the insertion strength information 104 to an electronic watermark pattern inserter 1 and to a shifting information pattern inserter 2. The electronic watermark pattern inserter 1 inserts an electronic watermark pattern 105, stored in an electronic watermark pattern storage unit 4, into the input picture 101. The key information pattern inserter 2 inserts the key information pattern 106, stored in the key information pattern storage unit 5, into a picture to be processed 102, having the inserted electronic watermark pattern, in accordance with the insertion strength information 104, to output the resulting picture.