An image processing apparatus, an image reading apparatus, an image forming apparatus and a computer-readable recording-medium for storing an image processing program are disclosed. The image processing apparatus can synthesize coded data encoded in accordance with JPEG 2000 standard from source and target images at a higher speed with less use of a memory. The image processing apparatus includes a search part and an objective block synthesis part. The search part searches coded data of the source image per predetermined independently processable block for an objective block corresponding to a designated synthesis area. The objective block synthesis part synthesizes detected coded data of the objective block of the source image and coded data of the objective block of the target image.