An apparatus for embedding a watermark into contents data includes a pattern generating device for generating bits representing a predetermined bit pattern corresponding to a watermark. A specified-bit detecting device operates for detecting bits in original picture data as specified bits into which a watermark can be embedded. A calculating device operates for calculating a desired bit pattern represented by the specified bits in response to the predetermined bit pattern and a specified bit pattern. The desired bit pattern can be converted into the specified bit pattern by given logical operation with the predetermined bit pattern. A mixing device operates for changing the specified bits to represent the desired bit pattern to convert the original picture data into watermark-embedded picture data.