An image processing apparatus includes a condition setting unit with which a user sets a post-quantization condition. A partial decoding and decompression unit partially decodes and decompresses coded data according to the post-quantization condition set by the user, thereby reproducing image data the same as that reproduced by decoding and decompressing coded data after post-quantization processing in accordance with the post-quantization condition. An image data displaying unit displays on a display unit the image data reproduced by the partial decoding and decompression unit.