A planar light source device 200 includes a light source 10 and a reflection member 30. The light source 10 emits a light beam 10a having directivity. The reflection member 30 has a box shape including a single bottom-plate part 30e, side-plate parts 30a, 30b, 30c and 30d connected to the bottom-plate part 30e, and an opening part 30f facing the bottom-plate part 30e. The box shape has inner surfaces which are reflection surfaces. The bottom-plate part 30e has a quadrilateral shape including two facing short sides and two facing long sides. The light beam 10a enters an inside of the box shape of the reflection member 30 through a side of the side-plate part 30a connected to the short side of the bottom-plate part 30e, travels in a direction of the long side of the bottom-plate part 30e while being reflected at the side-plate parts 30c and 30d connected to the long sides of the bottom-plate part 30e, and is emitted through the opening part 30f. The reflection surfaces of the side-plate parts 30c and 30d connected to the long sides of the bottom-plate part 30e include regions at which the light beam 10a is specularly reflected and regions at which the light beam 10a is diffused and reflected.