A mask blank substrate for a photomask is chucked on a mask stage of an exposure apparatus. A main surface, on the side where a thin film for a transfer pattern is to be formed, of the mask blank substrate has a flatness of 0.3 μm or less in a 142 mm square area including its central portion and has a convex shape being relatively high at its central portion and relatively low at its peripheral portion. The difference upon fitting, to the main surface of the mask blank substrate, a virtual reference main surface, having a spherical shape in a 132 mm square area, of a virtual reference substrate is 40 nm or less.