To provide a method of manufacturing a photomask blank which lessens the warpage of a substrate by film stress (relieves the film stress) and improves the density and denseness of the films.

The method of manufacturing the photomask blank by forming the light shielding films or light translucent films on a transparent substrate comprises forming the light shielding films or light translucent films by depositing the light shielding films or light translucent films by a sputtering method on the substrate 6 and simultaneously irradiating the film materials under deposition on the substrate 6 with the ions formed by an ion former (hereafter an ion source 11) separately included in a deposition chamber.