Abstract: A light source optical system for endoscopes includes an elliptical condensing mirror projecting a bright spot of the light-emitting section of a lamp and a relay optical system transmitting the projected image of the bright spot to the entrance end face of a light guide. The elliptical condensing mirror is designed to satisfy the following condition:2.0 mm<F<16.0 mmwhere F is the focal length of the elliptical condensing mirror, which is expressed by F=.beta..sup.2 /(2.alpha.) where (the major axis of an ellipse)/2=.alpha. and (the minor axis of the ellipse)/2=.beta.. In this way, a light beam from the light-emitting section can be efficiently condensed on the entrance end face of the light guide and the entire light source optical system can be constructed to be compact.