An image processing apparatus performs variable-length compression on the image data of each block included in one page, and adds a bit to the resultant code data of each block to make it have a constant data length. The apparatus stores the data length of the added bit. On the other hand, the apparatus stores a variable-length code from which the added bit is erased in a storage. If it is determined that respective bits are added to all blocks to make them have the constant data length, the apparatus adds respective bits of different data lengths to variable-length codes read from the storage, thereby converting the data lengths of the blocks into a shorter constant data length than the constant data length.