A plane source of light (back light) of the type of emitting light from both surfaces featuring a good light utilization efficiency and without permitting the external light to transmit through a display device even when it is combined with display panels of the transmission type. Light is introduced from a point source of light into a light guide plate. Many deflecting patterns of a triangular shape in cross section are formed in a recessed manner in a surface (pattern surface) on the side opposite to a light-leaving surface of the light guide plate. A prism sheet forming a plurality of arcuate prisms is arranged on a position facing the pattern surface of the light guide plate. Light propagating through the light guide plate is totally reflected by the deflecting pattern and goes out from the light-leaving surface nearly perpendicularly thereto. Light propagating through the light guide plate transmits through the deflecting patterns, goes out aslant, and is bent by the prisms into a direction nearly perpendicular to the pattern surface as it transmits through the prism sheet.