A photomask designing method used in a lithography process, the
lithography process comprises illuminating light on a photomask and
converging the light which has passed through the photomask on a
photosensitive substrate via a projection optical system, the photomask
designing method comprises acquiring a transmittance characteristic of
the projection optical system, the characteristic varing depending on a
difference in optical paths of light in the projection optical system,
the light passing through the projection optical system, and acquiring
mask bias of the photomask by use of the transmittance characteristic of
the projection optical system.