A light collective optical system includes a single optical member having a first reflecting surface and a second reflecting surface. In this case, the first reflecting surface partly has a first transmissive-refractive surface and the second reflecting surface partly has a second transmissive-refractive surface. The first transmissive-refractive surface and the second transmissive-refractive surface are nearly coaxially arranged. Whereby, it is possible to provide the light collective optical system which is capable of illuminating an object to be illuminated with high illuminance and is compact and a light source device using the light collective optical system.