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 .