An image processing apparatus compresses multi-level images created by a computer or the like at a high compression rate. To achieve this, the image processing apparatus includes an ADCT compression unit for compressing input multilevel image data for every block of data (the blocks having a predetermined size) by an irreversible compression method, and a reversible compression unit for compressing the input multilevel image data for every block by a reversible compression method. Selection between output data from the two compression units on the basis of data having a predetermined level and compressed by the reversible compression unit.