A photomask is washed and at least one physical amount of transmittance and phase difference of the photomask, dimension of a pattern, height of the pattern and a sidewall shape of the pattern is measured. After this, the two-dimensional shape of a borderline pattern previously determined for the photomask is measured. Lithography tolerance is derived by performing a lithography simulation for the measured two-dimensional shape by use of the measured physical amount. Then, whether the photomask can be used or not is determined based on the derived lithography tolerance.