The watermark insertion section divides the quantized DCT coefficients into at least two data streams, and modifies values of AC components of the data streams, so as to embed the watermark information in each block of the image information with the DCT coefficients quantized. The modification is performed as follows. The sum of a plurality of data of each data stream (or the sum of a plurality of data of a prescribed portion of each data stream) is divided by 2. Then, a value of an AC component of each data stream is modified such that the value of each remainder (the value of the LSB) has a prescribed relationship with (for example, is equal to) the value of the watermark information. Specifically, the value of the first non-zero AC component, from the trailing end of each data stream, is processed by addition of 1 or subtraction of 1.