A method for manufacturing a photomask based on design data includes the steps of forming a figure element group including a figure element in a layout pattern on the photomask and a figure element affecting the figure element due to the optical proximity effect, adding identical identification data to a data group indicating an identical figure element group, estimating an influence of the optical proximity effect on the figure element group, generating correction data indicating a corrected figure element in which the influence of the optical proximity effect is compensated for at the time of exposure, creating figure data by associating data having the identical identification data with correction data having the identical identification data, and forming a mask pattern on the photomask using figure data. Thus, the computation time for correction of the layout can be reduced, thereby reducing the production time of the photomask.