A lithography mask production method includes (a) forming, in a reflection layer of a blank substrate, a reference pattern used as a reference in reflectivity measurement and a reflection pattern used for lithography; (b) measuring a reflectivity Rref of the reflection layer at the reference pattern and a reflectivity RLS of the reflection layer at the reflection pattern; and (c) determining an effective width of the reflection layer at the reflection pattern based on the reflectivity Rref and the reflectivity RLS.