The present invention provides an additional-watermark embedding apparatus for embedding predetermined additional data into original data. The additional-watermark embedding apparatus includes an adder for summing the original data and the additional data, a first amplifier for amplifying the original data according to a predetermined non-linear input-output characteristic, a second amplifier for amplifying the summed data supplied from the adder according to a predetermined non-linear input-output characteristic, a subtractor for taking the difference between the amplified data obtained by the second amplifier and the amplified original data obtained by the first amplifier, and an embedder for embedding modified additional data which is obtained by modifying the waveform of the additional data according to the original data and which is supplied from the subtractor into the original data.