To provide a light sensor preventing an excimer lump of a simple structure from being damaged by a undesired load and suppressing the variation of measured values by the convection of the air of which the oxygen concentration is changed, and to provide a lamp unit provided with the same.

In the lamp unit consisting of the excimer lamp 5 emitting ultraviolet rays, the light sensor 6 receiving the ultraviolet rays and a box body 4 housing and holding at least the light sensor 6, the light sensor 6 is provided with a light detector 61 detecting visible light, a cylindrical holding body 63 annexed to the light detector 61 and a cylindrical or column-shaped moving body 64 movably held in the holding body 63 and installed in the excimer lamp 5 in a freely abutting way, and a permeating member 645 permeating the ultraviolet rays from the excimer lamp 5 and a converting member 646 converting the ultraviolet rays which have transmitted the transmitting member 645 to the visible light are surrounded with the light detector, the holding body and the moving body.