Mask blank substrate and method for producing same

A mask blank substrate which has two main surfaces as a first main surface and a second main surface and in which each of the main surfaces has a rectangular shape of 152 mm or more×152 mm or more, wherein

• when a rectangular region which is surrounded by four sides that are located at 5 mm from four sides of the main surface in an inward direction and are parallel to the four sides of the main surface and in which an intersection of diagonal lines of the main surface forms a center is defined,
• a flatness of the rectangular region on at least one of the first main surface and the second main surface is 100 nm or less, and
• when a surface shape (S) of the rectangular region is separated into an X direction component (Sx) extending along one of the four sides of the main surface and a Y direction component (Sy) extending along another side orthogonally crossing the one side, and
• of a PV value (PVx) that is a difference between a maximum height and a minimum height of the X direction component and a PV value (PVy) that is a difference between a maximum height and a minimum height of the Y direction component, the smaller is PVmin and the larger is PVmax,
• a ratio given by PVmin/PVmax (PV ratio) is ⅓ or less.