To provide a flat light source device 19 capable of improving the contrast on a display of a reflection liquid crystal panel 12.

A light-emitting diode 20 of a blue-color light having a luminous peak at a wavelength of 460 nm is used as a light source. A light guide plate 23 has main faces 25, 26, one of which 25 faces a reflection liquid crystal panel 12. The light from the light-emitting diode 20 is incident to the light guide plate 23, and emitted from one of the main faces 25 of the light guide plate 23 to the reflection liquid crystal panel 12. An anti-reflection film 28 with reflection spectrum characteristics showing the least reflectance at a wavelength of 580 nm up to a wavelength of 600 nm is provided at the one of the main faces 25 of the light guide plate 23. Contrast is improved by optimizing the relation between the luminous peak of the light-emitting diode 20 and the reflection spectrum characteristics of the anti-reflection film 28.