Abstract: Since a peripheral part of the sapphire plate 2 is airtightly thermally crimped to an end face (window peripheral part) of a kovar metal tube 1A surrounding a light entering window via a seal ring 7 made of aluminum, a lattice defect is prevented from occurring on the sapphire plate 2 due to diffusion of impurities on the sapphire plate 2. As a result, the generation of phosphorescence or fluorescence from the sapphire plate 2 is prevented in detecting an ultraviolet ray, and dark noise is reduced.