Sample A is produced by sequentially forming a first insulating film of SiO2 and a reflective film on a sapphire substrate. Sample B is produced by sequentially forming a first insulating film of SiO2, a reflective film, and a second insulating film of SiO2 on a sapphire substrate. In both samples A and B, the reflectance of the reflective film was measured at a wavelength of 450 nm before and after heat treatment. Heat treatment was performed at 600° C. for three minutes. As shown in FIG. 1, in Al/Ag/Al where Al has a thickness of 1 Å to 30 Å, Ag/Al where Al has a thickness of 20 Å, and Al/Ag/Al/Ag/Al where Al has a thickness of 20 Å, the reflectance was 95% or more, which is equivalent to or higher than that of Ag even after the heat treatment.