1. A pattern size correcting device for correcting size variation that occurs in an actual device pattern formed in photolithography depending on open area ratio of light transmitting areas around the actual device pattern, said device comprising: a testing photomask having a test pattern; a quantifying unit that quantifies, using said testing photomask, the size variation in the test pattern as a function of distance and in relation to the open area ratio; an open area ratio calculating unit that divides an exposure area having the plural actual device patterns into a plurality of correction areas and calculates the open area ratio of the respective correction areas; and a data correcting unit that inputs the open area ratio calculated by said open area ratio calculating unit to a result of the quantification, calculates the size variations of the actual device patterns in the respective correction areas and corrects design data of the actual device patterns based on the calculation.