A decompressing apparatus comprises a first decompressing unit, a second decompressing unit which can decompresses encoded data that the first decompressing unit does not support, an identifying unit which identifies a type of each encoded block, a generating unit which, if encoded block of interest is a type that can be decompressed by the first decompressing unit, stores the encoded data in a memory, and, if not, stores predetermined encoded data that can be decompressed by the first decompressing unit in the memory, and control unit which controls the first decompressing unit to decompress the encoded data in the memory to generate tentative image data, then controls the second decompressing unit to decompress the encoded data that cannot be decompressed by the first decompressing unit, and updates the tentative image data with the decompression result.