Provided is a reflective mask capable of reducing out-of-band light when transferring a prescribed pattern onto a wafer by exposure using EUV light in a process of manufacturing a semiconductor device. The mask blank substrate is provided with a base film on a substrate, the base film is formed with a material having a refractive index smaller than the substrate over a wavelength range of not less than 190 nm and not more than 280 nm, and reflectance of the base film arranged on the surface of the substrate is smaller than the reflectance of the substrate over a wavelength range of not less than 190 nm to not more than 280 nm.