An image processing apparatus which detects a plurality of image areas included in an original plate image to extract a minimum rectangular area including the plurality of image areas, specifies maximum eight line segments acquired by extending four sides of the rectangular area toward corresponding four sides of the original plate image, respectively, extracts pixels existing in a neighborhood area including each of the specified line segments, and determines that the rectangular area and the minimum rectangular area including the extracted pixels are the areas of the original image. With the configuration, the image processing apparatus can accurately decide only an original area even if edge information of an original extracted from a read image is unclear.