PURPOSE: To improve the uniformity of the intensity level of the scattered light while keeping its intensity level high, by providing light sources at both end surfaces of a bar-like transparent body and setting desired luminosity conditions therefor.

CONSTITUTION: One side surface of the surfaces of the bar-like transparent body is given as a light-scattering surface 1a, and the surface thereof that opposes the light- scattering surface 1a is given as a light-emitting surface 1c. The remaining side surface thereof is given as a light reflection surface, and light sources 2 are disposed at both end surfaces 1d of the transparent body, whereby the light-scattering surface is used as a luminous surface. In this case, the following conditions are satisfied. Namely, light-scattering portions 1e and light-non-scatering portions if co-exist in the light- scattering surface 1a. And the proportion of those light-scattering portions 1e varies relative to the position of the transparent body as viewed lengthwise thereof and simultaneously is symmerical about the center thereof as viewed lengthwise thereof. Assume now that the proportion of the light-scattering portions is represented by F, the length of the transparent body by L, and the longitudinal position thereof as measured from the one-side end surface by p, then the above variation in proportion satisfies the following equations. (1) F=F0+k.p (0≤p<L/2), (2) F=FL/2-K.(p-L/2) (L/2≤p≤L).