To provide a light source device having a small and simple focus optical system that makes incident light emitted from a solid-state light-emitting element to be efficiently incident on a light guide making up a lighting system.

The light source device includes a condensing lens system having positive power, a transparent columnar reflective member, and the solid-state light-emitting element. The condensing lens system at least includes a plano-convex lens having a flat surface. The plano-convex lens is disposed such that the flat surface side of the plano-convex lens faces an emission surface of the solid-state light-emitting element across an air gap so that an image of the emission surface of the solid-state light-emitting element is created as a virtual image. The columnar reflective member is disposed on the emission side of the condensing lens system so that the columnar reflective member is optically coaxial with the condensing lens system.