1. An image processing apparatus, comprising: an image input unit configured to input a document image; a reducing unit configured to generate a reduced document image by reducing the document image; an edge image generating unit configured to generate an edge image by extracting at least one edge of the reduced document image; and an inclination detecting unit configured to detect an inclination of the document image based on an inclination of a straight line representing an edge of the edge image, wherein said inclination detecting unit detects the straight line representing the edge of the edge image by performing a Hough transformation, and detects the inclination of the document image based on an inclination of the straight line, and wherein said inclination detecting unit is operable, if a plurality of straight lines that have different inclinations are detected, to determine a smallest inclination among the inclinations of the plurality of straight lines to be the inclination of the document image.