A translucent cover (9) is so provided as to cover a high pressure sodium lamp (11) adapted to radiate at least visible light and rays of light in the wavelength range of 300 nm to 400 nm. A photocatalytic film (13) which is capable of preventing generation of interference between visible light is formed on either one of or both the inner side and outer side of the translucent cover (9). The photocatalytic film (13) contains as the principal component titania (TiO2) of which the peak wavelength for visible light transmittance is nearly the same as the peak wavelength for visible light radiated from the high pressure sodium lamp (11). Ultraviolet light radiated from the high pressure sodium lamp (11) enhances oxidation and decomposition of substances performed by the photocatalytic film (13), thereby preventing grime and contaminants from adhering to the translucent cover (9). Even if the thickness of the photocatalytic film (13) is increased in order to make the photocatalytic function more effective, visible light radiated from the high pressure sodium lamp (11) effectively passes through the photocatalytic film (13) at the peak wavelength so that decrease in the illuminating efficiency is prevented.