To improve the accuracy of transmittance of a gray scale mask beyond the resolution of a mask drawing device when the gray scale mask is manufactured.

A design shape area A1 is defined according to a predetermined transmittance. A first opening area A2 is defined by rounding the design shape area A1 according to the resolution of the mask drawing device while maintaining the similarity relation with the design shape area A1. A difference is calculated by subtracting the area of the first opening area A2 from the area of the design shape area A1. An area adjusting area A4 corresponding to the difference is disposed adjacently to the first opening area A2 so that the width W1 is integer times the minimum drawing unit of the mask drawing device, thereby defining a second opening area A5 consisting of the first opening area A2 and area adjusting area A4. An opening is formed in a light shielding film along the second opening area A5 by the mask drawing device.