1. A method for manufacturing a photomask in which are arranged a light transmitting pattern portion and a light shielding pattern portion for forming an optical image pattern on the surface of a substrate, comprising: calculating a pattern area ratio, which is a ratio of the light transmitting pattern portion or the light shielding pattern portion to an area of the photomask from the design data of a given layout pattern of the photomask, and a pattern density, which is a ratio of said light transmitting pattern portion or light shielding pattern portion within said region to the area of the region extracted from the given layout pattern; estimating from the calculated pattern area ratio and the pattern density the size of a pattern formed in the case where the pattern is formed on the photomask by using the design data of the given layout pattern; and imparting the amount of correction to the design data of the given layout pattern based on the estimated pattern size.