An illumination optical system with which favorable light distribution is obtained even when the in-water angle of view is wide and which is suitable for implementation in small diameter medical endoscopes. The illumination optical system is equipped with a light guide provided at the tip of the inserted section of the endoscope and an illumination lens disposed adjacent to the light guide on the object side. The end face of the leading end of the light guide has a non-circular cross-sectional shape with a major axis and a minor axis. The light guide-side surface of the illumination lens is a concave surface having an effective area that is equal to or greater than the cross-sectional shape of the light guide. The concave surface is a toric surface, an anamorphic surface or an elliptical surface and satisfies the following conditional formulas:0.3 < (RS)/(LS) < 0.7 --- (1)1 < (RL/LL)/(RS/LS) < 5 --- (2)(LL) is the maximum length of the end face of the leading end of the light guide in the major axis direction. (LS) is the maximum length of the end face of the leading end of the light guide in the minor axis direction. (RL) is the absolute value of the central radius of curvature of the concave lens surface in the major axis direction. (RS) is the absolute value of the central radius of curvature of the concave lens surface in the minor axis direction.La présente invention concerne un système optique déclairage qui permet dobtenir une distribution favorable de la lumière, même lorsque lange de vue dans leau est large et qui est apte à être mis en œuvre dans des endoscopes médicaux de petit diamètre. Le système optique déclairage est doté dun guide de lumière disposé sur la pointe de la section insérée de lendoscope et dune lentille déclairage disposée à proximité du guide de lumière sur le côté objet. La face dextrémité de lextrémité avant du guide de lumière présente une forme en section transversale non circulaire ayant un axe principal et un axe secondaire. La sur
