Provided are an image processing apparatus, image processing method and a computer-readable, non-transitory medium that can accurately detect document edges from a readout image. The image processing apparatus includes a first edge pixel detector for detecting a plurality of first edge pixels from an input image, a straight line detector for detecting a straight line from the first edge pixels, a classifying module for classifying the first edge pixels into on-line edge pixels and non-on-line edge pixels, a second edge pixel detector for detecting second edge pixels located between two of the on-line edge pixels, when there is any one of the non-on-line edge pixels between two of the on-line edge pixels, and a determining module for determining whether the non-on-line edge pixel represents a document edge, based on whether the two on-line edge pixels are connected by the second edge pixels via the non-on-line edge pixel.