To more simply synthesize or separate insertion signals with and from original signals.

A signal synthetic processing device for synthesizing at least one insertion signal with an original signal comprises: a mixture ratio determination part for determining a mixture ratio at which quantification data of the original signal is synthesized with quantification data of the insertion signal; a synthesizing number determination section for determining the number of synthesizing the insertion signal with the original signal according to the determined mixture ratio; an element determination section for determining a first element for synthesizing elements of the insertion signals among elements of the original signal according to the determined synthesizing number, and a second element for not synthesizing the elements of the insertion signal that is deemed as the same data amount as the first element; an embedding signal creation section for embedding a predetermined code element to be synthesized with the second element in the insertion signal to create an embedded signal; and a synthetic signal creation section for synthesizing the embedded signal with the original signal at the mixture ratio determined by the mixture ratio determination section to create a synthetic signal.