1. A data correcting apparatus for correcting drawing data representing a drawing pattern to be drawn in a quadrangular drawing area of a drawing subject, based on an ideal position and an actual position of said drawing area on said drawing subject, an actual outline being an outline of said drawing area in said actual position, and an ideal outline being an outline of said drawing area in said ideal position, comprising: a first data corrector that corrects said drawing data to be first corrected drawing data, where a first corrected outline being an outline of said drawing area represented by said first corrected drawing data is a parallelogram which has two neighboring sides that are the same as two neighboring sides of said actual outline, said parallelogram having a shape closer to that of said ideal outline than said actual outline.