1. An image processing apparatus comprising: halftone frequency determining means for determining a halftone frequency of an image that has been read from a document by an image reading apparatus, the halftone frequency determining means comprising: flat halftone discriminating means for extracting information of density distribution per segment block consisting of a plurality of pixels, and discriminating, based on the information of density distribution, whether the segment block is a flat halftone region in which density transition is low, or a non-flat halftone region in which the density transition is high; threshold value determining means for determining a threshold value by using an adjusting value that is predetermined in accordance with a reading property of the image reading apparatus with respect to respective color components, the threshold value being for use in extraction of a feature of density transition between pixels; extracting means for extracting, by using the threshold value determined by the threshold value determining means, the feature of density transition between pixels of the segment block which the flat halftone discriminating means discriminates as the flat halftone region; and halftone frequency estimating means for estimating the halftone frequency, based on the feature extracted by the extracting means.