1. An apparatus for determining a skew angle of an input document that contains at least a plurality of lines of text characters and a non-text portion, said apparatus comprising: input means for generating an image of the input document; detection means coupled to said input means for detecting a skew angle of said plurality of lines of text characters, comprising means for extracting at least one region including only text lines from the document with respect to a plurality of scanning lines which across the document image at selected angles that vary intermittently from a reference angle, means for computing complexity at each of angles, means for finding out a characteristic parameter in a distribution of the parameters in the region, and means for determining a corresponding angle, at which said characteristic parameter appears, as the skew angle of the input document; and output means coupled to said detection means for generating a signal indicative of the skew angle thus determined.