A distortion correction device performs pre-processing on an input image data of a double page spread document or the like and produces an edge image. Upper and lower ends of the document are detected from the produced edge image. From the detected upper and lower ends of the document, a relative position of an imaging unit with respect to the document is detected. An approximate distance from the imaging unit to the document, the relative position of the imaging unit and information about the ends of the document are used to calculate respective heights of points on the document. The input image data then undergoes geometric conversion correction by means of information about the heights. In this way, the distortion correction device can be provided capable of easily and accurately correcting distortion of the image due to nonuniform height of the surface of the object, i.e., document, regardless of imaging conditions.