A planar light source device includes a primary light source (1), a light guide (3) which leads the light emitted from the primary light source (1), and having a light incident face (31) to which the light emitted from the primary light source (1) comes in, and a light emitting face from which the led light goes out, a light deflection element (4) arranged adjacent to the light emitting face (33) of the light guide (3), and a polarization separation element (6) disposed on the side of a light outgoing surface (42) of the light deflection element (4), and having a function of transmitting one of polarization components of the incident light and reflecting the other of the polarization components. The full width at half maximum of the luminance distribution of the incident light upon the polarization separation element (6) in an XY plane parallel to the travel direction of the light in the light guide (3) is 25.degree. or less. The reference symbol 7 indicates a liquid crystal display element.