There is provided a light source device which can miniaturize a two-dimensional image display device as small as possible. The light source device is provided with three coherent light sources (11a), (11b), and (11c) corresponding to red, blue, and green; prisms (12a) and (12c) for reflecting lights emitted from the coherent light sources (11a) and (11c); and a diffraction part (20) comprising a single volume hologram on which plural gratings are multiply-formed, which gratings diffract the light emitted from the coherent light source (11b), and the lights that are emitted from the coherent light sources (11a) and (11c) and reflected by the prisms (12a) and (12b) so that these lights propagate in the same optical path.