A reference point determinator determines, as a reference point, the center of each pixel where a sloped line is located. A candidate line generator generates candidate lines each connecting between a point in a pixel having the start point of the sloped line extracted by an endpoint extractor, and a point in a pixel having the end point of the sloped line. A candidate line selector selects a candidate line having the smallest sum total of distances between the candidate line and the reference points from the candidate lines generated by the candidate line generator. A data corrector corrects the sloped line to the candidate line selected by the candidate line selector.