1. A correcting device for a perspective transformed document image, said correcting device comprising: a horizontal vanishing point determining unit, for detecting a horizontal vanishing point of the perspective transformed document image; a vertical vanishing point determining unit, for detecting a vertical vanishing point of the perspective transformed document image; and a perspective transformation correcting and converting unit, for correcting the perspective transformed document image; wherein the horizontal vanishing point determining unit comprises a direct horizontal line segment detecting unit, an indirect horizontal line segment detecting unit and a horizontal vanishing point detecting unit, and wherein the horizontal vanishing point detecting unit detects a horizontal vanishing point in accordance with direct horizontal line segments detected by the direct horizontal line segment detecting unit and indirect horizontal line segments detected by the indirect horizontal line segment detecting unit; and wherein the horizontal vanishing point detecting unit comprises: a horizontal line segment intersection point clustering unit, for clustering point collections formed by intersection points of two horizontal line segments in a collection formed by direct horizontal line segments and indirect horizontal line segments to obtain a plurality of clusters; a candidate horizontal vanishing point selecting unit, for selecting each cluster as a candidate horizontal vanishing point and obtaining a weight coefficient of each candidate horizontal vanishing point; a horizontal direction perspective projection analyzing unit, for performing perspective projection analysis in a horizontal direction on the document image with regard to the candidate horizontal vanishing points and obtaining another weight coefficient of each candidate horizontal vanishing point; and a horizontal vanishing point synthesis analyzing unit, for selecting a final horizontal vanishing point in accordance with the weight coefficient and the another weight coefficient of each of the candidate horizontal vanishing points.