A pattern size correcting device includes: a testing photomask (1) having a test pattern; a quantifying unit (2) that quantifies, using the testing photomask (1), size variation in the test pattern as a function of distance and in relation to an open area ratio; an open area ratio calculating unit (3) that divides an exposure area having a plurality of actual device patterns into a plurality of correction areas and calculates the open area ratio of the respective correction areas; a data correcting unit (4) that inputs the open area ratio calculated by the open area ratio calculating unit (3) into a result of the quantification that uses the photomask (1), calculates 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; and a proximity effect correcting unit correcting a proximity effect. This correcting device enables quantitative estimation of size variation occurring in a pattern exposed in lithography and easy and accurate correction of pattern size based on the estimation.