The invention relates to a light source optical system for endoscopes which is compatible with various viewing modes, prevents an associated light source apparatus from growing bulky, and makes sure brightness. The light source optical system comprises, in order from the light source 1, the collective optical system 2 for collecting light from the light source, the magnification conversion optical system 3 for reducing a pupil magnification, and the positive lens group 4 for collecting light from the magnification conversion optical system 3, and satisfies Condition (1) with respect to the principal point-to-point distance d of the collective optical system 2 and magnification conversion optical system 3, the rear focal length f2B of the collective optical system 2 and the front focal length f3F of the magnification conversion optical system 3.