added contents data alternatively a signal representing the strength of the embedded watermark may be included in the watermark added contents data according to a first example the left upper corner pixel in every frame represented by the watermark added contents data is loaded with a signal representing the strength of the embedded watermark according to a second example a header of the watermark added contents data is loaded with a signal representing the strength of the embedded watermark in these cases the signal representing the strength of the embedded watermark is transmitted together with the watermark added contents data when the strength of the embedded watermark relative to the original picture is the smallest the pixel specified bit detector 47 sets the number n to 1 and therefore designates and selects only the lowest bit among 8 bits per luminance pixel as a bit in which a fragment of the watermark is embedded as shown in fig5 when the strength of the embedded watermark relative to the original picture is the second smallest the pixel specified bit detector 47 sets the number n to 2 and therefore designates and selects only the two lower bits among 8 bits per luminance pixel as bits in which a fragment of the watermark is embedded as shown in fig6 when the strength of the embedded watermark relative to the original picture is the third smallest the pixel specified bit detector 47 sets the number n to 3.