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.