1. An image processing apparatus for determining tilt angles of an image, the image processing apparatus comprising: a Hough transformation unit that transforms coordinates of points in the image into a distance ρ and an angle θ, and casts votes to a vote table including the distance ρ and the angle θ as headers to extract straight lines in the image; an extraction unit that extracts a predetermined number of points having numbers of votes ranked high from the vote table; and a tilt angle determination unit that determines an extraction angle θx to achieve maximum consistency with angles θ at the predetermined number of points, and determines the tilt angles of the image based on the determined extraction angle θx; wherein the extraction unit masks an area around each of extracted points in the vote table at a time of extracting the predetermined number of points having the numbers of votes ranked high, to prevent other points in a masked area from being extracted from the vote table.