In a method of joining image data divided into a plurality of parts each having an overlap region with the other at a border line portion into a single image for output, an image processing apparatus joins the plurality of partial images having overlap regions L at border line portions. The image processing apparatus extracts a joining line connecting pixels with small density difference between partial images from the overlap region, and joins the partial images with each other along the joining line. The image processing apparatus extracts a line connecting pixels with small density difference with adjacent pixels from the overlap region of the partial images as a joining line upon extracting the joining line. The density difference at the joining border is minimized in order to make the joining line look more natural, and make offset of images in the overlap region less recognizable.