1. A mask pattern design method comprising the steps of: dividing design layout data for a pattern into a plurality of regions and extracting any region wherein transfer dimensions obtained from a transfer simulation of said pattern from the plurality of regions exceeds a predetermined allowance range; setting a process window of which a plurality of transfer conditions of the pattern data from the region extracted by said process are each changed, and computing transfer dimensions obtained from a transfer simulation with each transfer condition with the process window; and extracting the transfer conditions wherein the transfer dimension obtained from the transfer simulation with each transfer condition with said process window exceeds a predetermined allowance range, and computing yield from an occurrence probability regarding the transfer condition.