A photomask manufacturing method. A pattern dimensional map is generated by preparing a photomask in which a reflective layer formed on a substrate and an absorber pattern is formed on the layer. A reflection correction coefficient map is generated by dividing a mask region, where the absorber pattern is formed, into a plurality of subregions, and determining a reflection correction coefficient for each subregion. The reflection correction value of each subregion is calculated based on the dimensional difference indicated in the pattern dimensional map and the reflection correction coefficient of each subregion. A reflection coefficient of each reflective layer region corresponding to each subregion is changed based on the reflection correction value.