Abstract: A magnetron sputtering apparatus for forming a thin metal film on one of the major surfaces of a light-transmitting disc-shaped substrate as a disc substrate for an optical disc as mutually intersecting magnetic fields are applied by a magnetic field application unit provided at back of a target. The apparatus includes a center mask tightly contacted with the outer peripheral portion of one major surface of the disc substrate on which the thin film is formed for masking the center portion of the substrate and an outer peripheral mask tightly contacted with the outer peripheral portion of the one major surface of the disc substrate on which the thin film is formed for masking the outer peripheral portion. The outer peripheral mask is separated from and independent of the center mask.

Abstract: A mesh belt type blackening lehr comprising: a discharge duct provided at a predetermined position of a ceiling of a blackening zone, the temperature of which is kept at a blackening temperature. The blackening zone is divided into a charge side region and a discharge side region. The charge side region is kept in an atmosphere of mixture gas of air and humidification exothermic type gas so as to perform first processing for forming an oxide film consisting of Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, and FeO. The discharge side region is kept in an atmosphere of humidification exothermic type gas so as to perform second processing for reducing Fe.sub.2 O.sub.3. Thus, an oxide film, consisting of mainly Fe.sub.3 O.sub.4, is formed on the surface of the shadow mask.