By introducing a gas containing a reducing organic gas into the discharge space, the protective layer is exposed to the reducing organic gas. Then, the reducing organic gas is exhausted from the discharge space. Then, a discharge gas is enclosed to the discharge space. The protective layer includes a base film made of magnesium oxide, and a plurality of metal oxide particles dispersed over the base film. The metal oxide particle includes at least a first metal oxide and a second metal oxide. Moreover, the metal oxide particle has at least one peak in an X-ray diffraction analysis. The peak is located between a first peak in the X-ray diffraction analysis of the first metal oxide and a second peak in the X-ray diffraction analysis of the second metal oxide. The first peak and the second peal have the same plane orientation as the plane orientation indicated by the peak.