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, comprising: an ideal position determiner that determines an ideal position of said quadrangular drawing area based on a position of said quadrangular drawing area before said drawing subject is transformed, wherein an ideal outline is an outline of said quadrangular drawing area in said ideal position; an actual position determiner that determines an actual position of said quadrangular drawing area based on the position of said quadrangular drawing area after said drawing subject is transformed, wherein an actual outline is an outline of said quadrangular drawing area in said actual position; a first data corrector that corrects said drawing data to be first corrected drawing data, where a first corrected outline, comprising 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, one of said two neighboring sides of said corrected first outline overlapping a side of said ideal outline.