To obtain a hafnium oxide film low in attenuation coefficient and dense in film structure by forming the film by at least an ion-beam sputtering method and controlling the attenuation coefficient in a specified wavelength range below a specified value.

A hafnium oxide film is formed by an ion-beam sputtering method in a lower vacuum than normally. Namely, a substrate is set in a device chamber, and then the device is evacuated. In this case, the vacuum is controlled to 1×10-6 to 5×10-7 Torr which is lower than the ultimate vacuum (1×10-7 to 3×10-7 Torr). Since a minute amt. of moisture is left under such a low vacuum, a film is low in absorption loss (low in attenuation coefficient) is formed by the action of the moisture. The attenuation coefficient is decreased to ≤0.005 in the wavelength range from 230 to 300 nm, an optical film having such a hafnium oxide film is low in absorption loss, and the reflectance or transmittance is improved.