An apparatus for forming a protective layer of magnesium oxide on a front glass substrate (11) in an evaporation chamber (201) includes the following: oxygen outlet openings (222) for introducing oxygen into the evaporation chamber (201); water vapor outlet openings (210) for introducing water vapor into the evaporation chamber (201) from the downstream side in the transfer direction of the front glass substrate (11); a mass analyzer (224) for measuring the ionic strength of hydrogen and the ionic strength of oxygen in the evaporation chamber (201); and mass flow controllers (215) and (221) for controlling the introduction amount of the water vapor and the introduction amount of the oxygen, respectively, by the ionic strengths measured by the mass analyzer (224).