To provide an endoscope illumination optical system whereby improved insertability and reduced unevenness of light distribution can be realized. An illumination optical system 10 has a transparent resin member 1 and an illumination member 2 for emitting illumination light, the transparent resin member 1 having a through hole 4 in which an imaging member 5 is disposed, a non-through hole 3 in which the illumination member 2 is disposed, and an internal optical surface 7, the illumination light being incident on the transparent resin member 1 via the internal optical surface 7 and emitted toward an object from an outer surface 6 of the transparent resin member 1, the outer surface 6 of the transparent resin member 1 having a curved-surface region 8 through which the illumination light passes, the curved-surface region 8 having a boundary with the through hole 4 formed in the region, displacement of each point of the curved-surface region 8 from the boundary occurring in the direction away from the object, and the shape of the curved-surface region 8 in a cross section defined by a plane including the central axis of the through hole 4 satisfying conditional expressions (1) and (2) and being such that the inclination angle at each point in the curved-surface region 8 continuously and monotonically increases progressively away from the central axis.L'invention vise à proposer un système optique d'éclairage d'endoscope permettant d'améliorer la capacité d'insertion et de réduire l'irrégularité de la distribution de la lumière. Un système optique d'éclairage 10 comprend un élément de résine transparent 1 et un élément d'éclairage 2 pour émettre de la lumière d'éclairage, l'élément de résine transparent 1 ayant un trou traversant 4 dans lequel un élément d'imagerie 5 est disposé, un trou non traversant 3 dans lequel est disposé l'élément d'éclairage 2, et une surface optique interne 7.La lumière d'éclairage est incidente sur l'élément de résine transparent 1 par l'interm
