The illumination light emitted from the light source to the light distributing efficiently observation target, to improve the light distribution performance. Provided in the insertion portion of the endoscope apparatus, a light guide bundle 10 that guides the illumination light emitted from the light source, an illumination lens 11 for the light distribution in the observation target with illumination light emitted from the light guide bundle, The features, the illumination lens, has two inflection points P in the vertical plane including the optical axis, a first concave surface 11a which is arranged inside of the two inflection points, two of the inflection and a second concave surface 11b of the annular continuous to said first concave surface of the outer periphery on the outside of the point, the second concave surface faces the optical axis direction with respect to the outer peripheral edge of the light guide bundle providing an endoscope illumination optical system which is arranged to.光源から射出される照明光を効率よく観察対象に配光させ、配光性能を向上させる。内視鏡装置の挿入部に設けられ、光源から射出された照明光を導光するライトガイドバンドル１０と、該ライトガイドバンドルから射出された照明光を観察対象に配光する照明レンズ１１と、を備え、該照明レンズが、その光軸を含む縦断面において２つの変曲点Ｐを有し、２つの該変曲点の内側に配置される第１の凹面１１ａと、２つの前記変曲点の外側に前記第１の凹面の外周に連続する円環状の第２の凹面１１ｂとを有し、該第２の凹面が、前記ライトガイドバンドルの外周縁に対して光軸方向に対向するように配置されている内視鏡用照明光学系を提供する。
