The present invention is a spectral image processing method including the steps of selecting observed spectral information from a multispectral image of a scene, generating a constraint on a spectral distribution of an illumination and a constraint on a surface reflectance of an object using a basis vector of the illumination and a basis vector of the surface reflectance, generating an observation equation that takes as parameters a weight coefficient of the basis vector of the illumination and a weight coefficient of the basis vector of the surface reflectance using the observed spectral information, the constraint on the spectral distribution of the illumination, and the constraint on the surface reflectance of the object, and calculating the spectral distribution of the illumination and the surface reflectance of the object of the scene from the observation equation.