1. A method for compressing a multispectral image obtained by using band images of a subject captured in a wavelength range divided into a plurality of bands, comprising the steps of: performing logarithmic conversion of said multispectral image and segmentation of said multispectral image into a plurality of tile images to obtain logarithmically converted tile image data; performing principal component analysis on said logarithmically converted tile image data of respective tile images to obtain for each tile image a principal component number of sets of principal component vectors and principal component images based on the multispectral image; determining from said plurality of sets, for each tile image, an optimum principal component number of sets of optimum principal component vectors and corresponding optimum principal component images that optimally represent image information about the multispectral image; and expressing compressed image data for said multispectral image by means of at least said optimum principal component number of sets of optimum principal component images and optimum principal component vectors for each tile image.