The obtained gel films 5Gel-Pr0.22Y0.78 BCO+Pr0.02Sm0.02Y0.92Tm0.04Ba2Cu3O7-x—C, and 5Gel-Pr0.22Y0.78BCO+Pr0.02Sm0.02Y0.92Er0.04Ba2Cu3O7-x—C were immediately installed in a furnace filled with dry gas, the calcining was performed under a humidified pure oxygen atmosphere at 400° C. or lower with the profile illustrated in