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 each and every one of a plurality of shot regions having a transferred pattern formed by a single time of light exposure through a mask pattern and each and every one of a plurality of chip regions comprised in one of the shot regions, a numerical aperture of the mask pattern corresponding to the transferred pattern onto the target object based on an exposure layout of the plurality of shot regions while considering a first flare due to influence from the peripherals of the shot region and a second flare due to influence from the peripherals of the chip region; and a dimension calculation unit calculating a dimension of the mask pattern necessary for obtaining the transferred pattern of a predetermined dimension, using the numerical aperture calculated in the above; wherein the numerical aperture calculation unit calculates an average numerical aperture for the plurality of shot regions and chip regions, corrects a design value of the numerical aperture for the plurality of shot regions and chip regions for the calculation using the average numerical aperture, and calculates an effective value of the numerical aperture for the shot regions and chip regions for the calculation.