A signal processing apparatus includes an optical intensity detecting section that detects an intensity of light from a subject at a first wavelength and a second wavelength, where the first wavelength is different from the second wavelength, a pigment component identifying section that identifies a pigment component in the light from the subject, based on (i) a difference between a first absorptance of the subject at the first wavelength and a second absorptance of the subject at the second wavelength and (ii) a difference between the intensity at the first wavelength and the intensity at the second wavelength that are detected by the optical intensity detecting section, and an illumination light component identifying section that identifies an illumination light component in the light from the subject, based on a subject image obtained by image-capturing the subject and the pigment component identified by the pigment component identifying section.