A glass photomask substrate, being a square-shaped glass substrate having a pair of major surfaces, for a photomask having a patterned light-shielding film on one surface of the substrate and which in use is chucked at chucking portions in an exposure tool, wherein a said major surface of the substrate having the chucking portions is delineated by four peripheral sides (edges), and wherein, for two strip regions defined on the major surface near an opposed pair of said sides, such that each strip region spans between 2 mm and 10 mm inward of its side and extends parallel thereto but excludes end portions extending 2 mm inward from the longitudinal ends of the side, characterised in that the angle between normal lines to the least squares planes of the two strip regions is less than or equal to 10 seconds, and the defined height difference between the strip regions is less than or equal to 0.5 μm; said height difference of the strip regions being defined on the basis that, where: F1 is the least squares plane of one strip region, F2 is the least squares plane of the other strip region, F3 is the least squares plane for a major surface region coextensive with the major surface, excluding peripheral portions extending 2 mm inward from the four sides but including the two strip regions, and F3' is a plane parallel to plane F3 and disposed such that the center of a strip region-equivalent region of plane F1 and the center of a strip region-equivalent region of plane F2 both lie on the same side of plane F3'; then the defined height difference is the absolute value |D1-D2| of the difference between a distance D1 along a normal line drawn normal to the plane F3' from the center of the strip region-equivalent region of F1 to the plane F3', and a distance D2 along a normal line drawn normal to the plane F3' from the center of the strip region-equivalent region of F2 to the plane F3'. the angle between normal lines to the least squares planes of the two strip regions is less than or equal to 10 seconds, and the defined height difference between the strip regions is less than or equal to 0.5 μm; said height difference of the strip regions being defined on the basis that, where: F1 is the least squares plane of one strip region, F2 is the least squares plane of the other strip region, F3 is the least squares plane for a major surface region coextensive with the major surface, excluding peripheral portions extending 2 mm inward from the four sides but including the two strip regions, and F3' is a plane parallel to plane F3 and disposed such that the center of a strip region-equivalent region of plane F1 and the center of a strip region-equivalent region of plane F2 both lie on the same side of plane F3'; F1 is the least squares plane of one strip region, F2 is the least squares plane of the other strip region, F3 is the least squares plane for a major surface region coextensive with the major surface, excluding peripheral portions extending 2 mm inward from the four sides but including the two strip regions, and F3' is a plane parallel to plane F3 and disposed such that the center of a strip region-equivalent region of plane F1 and the center of a strip region-equivalent region of plane F2 both lie on the same side of plane F3'; then the defined height difference is the absolute value |D1-D2| of the difference between a distance D1 along a normal line drawn normal to the plane F3' from the center of the strip region-equivalent region of F1 to the plane F3', and a distance D2 along a normal line drawn normal to the plane F3' from the center of the strip region-equivalent region of F2 to the plane F3'.