PURPOSE: To protect Fresnel surfaces, to improve the durability thereof and to facilitate their maintenance by constituting a titled plate body wherein a Fresnel plate is sealed hermetically between two sheets of glass plates, the inside is evacuated to pressure lower than atmospheric pressure and the projecting parts of the Fresnel surfaces are in abutment on the glass plate.

CONSTITUTION: A Fresnel plate 20 is sealed hermetically between two sheets of glass plates 10 and 30 via a sealant 40 and cushion rubber 60, and the inside thereof is evacuated to pressure lower than atmospheric pressure so that the projecting parts of the Fresnel lenses are brought into abutment on the glass plate 10. Since the Fresnel lenses are protected by the plates 10, 30, durability is improved and maintenance is facilitated.