PURPOSE: To illuminate a diffusing plate uniformly, by providing a pair of reflective mirrors, which are arranged at prescribed angles to the optical axis of a projecting optical system and a plane including respective linear light sources symmetrically with respect to a plane, around a light source part.

CONSTITUTION: The light emitted from a light source 1a is transmitted through a filter 2a and is reflected by a reflective mirror 3 and reaches a diffusing plate 4. At this time, when a virtual image 1a' of the light source 1a due to the reflective mirror 3 is considered, the illuminance is higher in the position nearer to the virtual image 1a' on the diffusing plate 4 as shown in Fig. by a solid line (a) because the diffusing plate 4 is illuminated directly from the virtual image 1a' equivalently. The light emitted in the opposite direction from the light source 1a is transmitted through the filter 2a and is reflected by a reflective mirror 3' similarly, and therefore, an illumination distribution symmetric to the curve (a) is attained with respect to a virtual image 1a" of the light source 1a as shown in Fig. by an alternate long and short dash line (b). Consequently, the illuminance of an optional point P on the diffusing plate 4 is approximately constant as shown by a broken line (c).