PURPOSE: To provide the surface light source element which is suitable as a back illuminating means of a liquid crystal display element, etc., and emits light with uniform luminance from the whole surface, and also, whose efficiency is high.

CONSTITUTION: A side face 7 of a transparent light guiding body 1 is set as an incident surface and a light source 4 is allowed to adhere closely, and also, by a reflector 5, light of the light source 4 is guided into the light guiding body 1 without waste. An emitting surface 6 of the light guiding body 1 is smoothed and decreases a loss of an emitted light. To the emitting surface 6, an element 3 having a prism unit is allowed to adhere closely, and also, the opposite surface 9 of the emitting surface 6 of the transparent light guiding body 1 is constituted so that the part whose surface is roughened and a smooth part 8 exist alternately, and moreover, a ratio of the smooth part increases as it draws near the light incident surface 7. Also, the reflecting surface 2 is provided in the vicinity of the opposite surface 9.