A light source device includes: a plurality of first semiconductor light sources; a condenser lens; and a light guide having a prismatic shape in which an incident surface is on a first side of a central axis direction of the prismatic shape and an emission surface is on a second side of the central axis direction of the prismatic shape, a positional relationship between the plurality of first semiconductor light sources, the condenser lens, and the light guide is set such that a traveling direction of the light incident on the incident surface from the first semiconductor light sources via the condenser lens is perpendicular to a reflecting surface of the light guide in a plan view as viewed in the central axis direction of the prismatic shape, the incident light being reflected on the reflecting surface in the light guide.