1. An image compression apparatus comprising: a dividing unit that divides a bit width pixel value set in advance that indicates compressed image information into an invalid bit region and a valid bit region other than the invalid bit region that are replaced with a fixed value determined in advance; an image information conversion unit that makes a value of the valid bit region be a value of a bit corresponding to the pixel value and makes a value of the invalid bit region be the fixed value determined in advance and converts the compressed image information to conversion image information; and a compression unit that compresses the conversion image information.