An object of the present invention is to provide a mask blank substrate and the like that enables critical defects to be reliably detected as a result of reducing the number of detected defects, including pseudo defects, even when using highly sensitive defect inspection apparatuses that use light of various wavelengths. The present invention relates to a mask blank substrate that is used in lithography, wherein the power spectral density at a spatial frequency of 1×10−2 μm−1 to 1 μm−1, obtained by measuring a 0.14 mm×0.1 mm region on a main surface of the mask blank substrate on the side of which a transfer pattern is formed at 640×480 pixels with a white-light interferometer, is not more than 4×106 nm4, and the power spectral density at a spatial frequency of not less than 1 μm−1, obtained by measuring a 1 μm×1 μm region on the main surface with an atomic force microscope, is not more than 10 nm4.