To provide manufacturing methods of a mask blank and a mask for producing a thin film for a transfer pattern having a reflection index adjustment film on its surface with low defective rate, reducing surface reflection index of the thin film in a short wavelength range of 190 to 300 nm used for an exposure light source when a pattern is transferred or a pattern inspection of the mask, and reducing the number of defects.

In the manufacturing method of a mask blank having the thin film for forming a transfer pattern on a substrate, after forming a thin film on the substrate, an oxide film is formed by radiating ozone gas to a surface of the thin film. The thin film comprises a material containing tantalum, and the film structure of the thin film is amorphous. The concentration of the ozone gas is 80 vol.% or more. The ambient temperature when radiating the ozone gas is a room temperature to 300°C.