1. A signal processing method comprising: a division step of inputting plural image data and dividing at least one of the plural image data into plural small areas; a parameter extraction step of extracting a conversion parameter used for converting the small areas so that the small areas become similar to the other image data; a totaling step of totaling values indicating the degree of similarity found on the basis of the conversion parameter; and a similarity evaluation step of evaluating the similarity between the plural image data on the basis of the result of the totaling.