1. A photomask designing method comprising: dividing design pattern data into predetermined regions; obtaining a pattern perimeter for each of the divided regions; obtaining the pattern perimeter for an entire region of the design pattern data by repeating the obtaining the pattern perimeter for the each of the divided regions; obtaining a dimension conversion difference for the entire region of the design pattern data using the pattern perimeter for the entire region of the design pattern data and a correlation obtained in advance between a predicted pattern perimeter and a predicted dimension conversion difference; performing process proximity correction on the design pattern data using a value of the obtained dimension conversion difference; and creating exposure pattern data from the corrected design pattern data.