[The present invention provides] a light source device comprising: a light conductor which has a first light incident end surface, a second light incident end surface and a light emitting surface; a first primary light source and a second primary light source which are disposed respectively adjacent to the first light incident end surface and second light incident end surface; and a light deflecting element which has a light incident surface disposed facing the light emitting surface and a light emitting surface, wherein a directional light emitting functional part with an average inclination angle of 0.5 to 15.degree. is formed on at least one surface of the light conductor, i.e., the light emitting surface or back surface, a plurality of mutually parallel prism rows extending in a direction parallel to the first light incident end surface and second light incident end surface are formed on the light incident surface of the light deflecting element, [each of these] prism rows has a first prism surface on the side close to the first primary light source and a second prism surface on the side close to the second primary light source, the first prism surface has a first region and a second region that extend in the direction of extension of the prism rows, the vertical angle of the prism rows is 80 to 120.degree., the difference in the angle of inclination between the first region and second region is 5 to 20.degree., and the second region has a smaller angle of inclination than the first region.