Abstract: A polycrystalline thin film B consisting mainly of oxide crystal grains 20 which have a crystal structure of a Type C rare earth oxide represented by one of the formulas Y2O3, Sc2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, Lu2O3, and Pm2O3 formed on a film forming surface of a polycrystalline substrate A wherein grain boundary inclination angles between the corresponding crystal axes of different crystal grains in the polycrystalline thin film along a plane parallel to the film forming surface of the polycrystalline substrate are controlled within 30 degrees.