1. A mask pattern correction device used in pattern transfer, onto a target object for pattern transfer, of a mask pattern of a photomask under light exposure repeated multiple number of times to thereby form a transferred pattern, comprising: a numerical aperture calculation unit calculating, for every single target region, a numerical aperture of said mask pattern corresponding to said transferred pattern, based on a layout of a plurality of said target regions; and a dimension calculation unit calculating a dimension of said mask pattern necessary for obtaining said transferred pattern of a predetermined dimension, using said numerical aperture calculated in the above.