A data compression apparatus includes a thinning section that thins numerical values from a succession of values constituting compressed data to create first and second compressed data; a reversible compression section for the first compressed data; a detecting section that detects an edge portion of the image; and a non-reversible compression section that processes the second compressed data based on a result of the edge detection. When the detecting section detects no edge portion, the non-reversible compression section outputs a predetermined code of a number of bits. When the detecting section detects the edge portion, the non-reversible compression section outputs a first value based on image numerical value when involved with a predetermined positional relation expressed by a first number of bits and a second value expressed by a few number of bits than the first number when the image numerical value is not involved with a predetermined positional relation.