1. An image compression apparatus comprising: a central processor configured to execute as: 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 the value of a bit corresponding to a 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, wherein the compressed image information is image information read out and acquired by an image reading unit, wherein the dividing unit compares a pixel value of test image information in which a test image with the pixel value set in advance is read out and acquired by the image reading unit and the pixel value set in advance and divides the bit width pixel value set in advance into the invalid bit region and the valid bit region other than the invalid bit region that has a bit width with a value different from the bit width set in advance, and wherein the dividing unit acquires the bit width with a different value for each pixel value by comparing the pixel value of the test image information and the pixel value set in advance for the each pixel value generates a frequency distribution by calculating a number of pixel values that have bit widths with different values for the each bit width, and divides the pixel value into the invalid bit region and the valid bit region by determining the invalid bit region using the frequency distribution.