1. A method of evaluating a photo mask, comprising: measuring each dimension of a plurality of pattern portions of a mask pattern formed on a photo mask; obtaining an inter-pattern distance between the pattern portion and a pattern different from the pattern portion with respect to each of the pattern portions; obtaining a dimensional difference between the measured dimension of the pattern portion and a target dimension of the pattern portion with respect to each of the pattern portions; grouping the dimensional difference obtained for each pattern portion into a plurality of groups in accordance with the inter-pattern distance obtained for each pattern portion; obtaining an evaluation value based on the dimensional difference in each group with respect to each of the groups; and evaluating the photo mask based on the evaluation value.