A method for reducing oxygen and carbon components in high-purity fluoride such
as calcium fluoride and barium fluoride (a high-purity fluoride that is not limited
to calcium fluoride and barium fluoride), or in rare-earth fluorides such as ytterbium
fluoride and cerium fluoride used for an optical fiber, a coating material, and
so on. The oxygen and carbon components contained in the high-purity fluoride are
reduced by a fluorine gas treatment.