An image compression unit of the invention includes a preprocessing unit that performs preprocessing for converting a first bit string representing intensity of pixels of which image data is composed into a second bit string, a number of digits of which is smaller than a number of digits of the first bit string, based on a predetermined region included in the image data and a first attribute information for identifying an attribute of the predetermined region, with the preprocessing being associated with the predetermined region of the image data and a bit string conversion unit which converts the second bit string into a third bit string based on distribution of frequency of appearance of the intensity in the second bit string and the first attribute information such that, for the pixel having higher frequency of appearance, the number of bit planes on which data of the pixel appear is less.