To reduce the number of part items and power consumption and prevent the generation of heat and the lowering of service life of a light source.

A light source 6 for the back light of a display 5 is driven by a circuit part 8 and irradiates with the luminance proportional to the light receiving quantity of an optical sensor 9. A shading plate 11 is interlocked with a falling-type hologram combiner 2. When the hologram combiner 2 is put in a leveled state, the shading plate 11 is positioned between the optical sensor 9 and the light receiving aperture 10 to shut out the light from the external world and to act in such a way as to hinder the light from reaching the optical sensor 9. At the leveled time of the hologram combiner 2, the light receiving quantity of the optical sensor 9 is reduced by a large margin, so that the supply electric energy to the light source 6 is reduced by a large margin.