A photomask substrate with a substantially uniform thickness comprises: a first surface, which is a continuous curved surface whereon a mask pattern is to be formed; and a second surface. The first surface exhibits a square shape that comprises an opposing pair of first set sides and an opposing pair of second set sides and has support parts at end parts along the first set sides. When the photomask substrate is held such that the first surface is in a substantially vertical state, a reference plane that is parallel to a tangential plane of the first surface at the center point of the first surface is defined on the photomask substrate side that is closer to the first surface than to the second surface. At this time, a first distance in the thickness direction between the reference plane and the center point of the first surface is shorter than second distances in the thickness direction between the reference plane and the midpoints of the second set sides.