To reduce the light absorbing factor to light beams in a region of a wide range of wavelengths, especially the light beams in a blue color light wavelength region when an optical single crystal film is manufactured from a fused by body contg. a transition metal by the liq. phase epitaxial method.

An epitaxial film of the optical single crystal is formed on a single crystal substrate from the fused body contg. the transition metal by the liq. phase epitaxial method. The manufacturing method for the optical single crystal film is provided with a state for annealing the epitaxial film at a prescribed temp. in the atmosphere contg. ozone, a stage for raising the temp. of the epitaxial film to a prescribed temp. and a state for lowering the temp. of the epitaxial film. The epitaxial film is exposed to the atmosphere in which ozone is not substantially contained, in at least one of the temp. raising stage and the temp. lowering stage. Preferably, the transition metal is vanadium.