1. An image-region discriminating method comprising the steps of: based upon all image data within a specified region including a focused pixel that serves as a target for discriminating the feature of a region to which respective pixels on an image belong and a plurality of adjacent pixels located around the focused pixel, extracting an amount of feature indicating the feature of the region to which the focused pixel belongs, the step of extracting the amount of feature including the steps of extracting a degree of frequency from the specified region by obtaining a first sum of absolute values of the density-level difference between pixels adjoining in a line direction of the pixels within the specified region and a second sum of absolute values of the density-level difference between pixels adjoining in a direction orthogonal to the line of the pixels, and extracting a comparison difference value from the specified region by obtaining a difference between a maximum value and a minimum value of density-level of all the pixels within the specified region, wherein the amount of feature includes the degree of frequency and the comparison difference value; and discriminating the kind of the region to which the focused pixel belongs based upon the amount of feature.