An imaging optical system for realizing an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high in resolution is provided. The imaging optical system includes a first lens having a negative refractive power, a second lens having a positive refractive power, a diaphragm, and a third lens having a positive refractive power, which are disposed from the object side to the image side. . The distance between the image side principal point of the first lens and the object side principal point of the second lens is d , and the image side principal point of the first lens is imaged more than the object side principal point of the second lens. The signed distance between the principal points is d = − d , and the combined focal length of the first lens and the second lens is f12, and d <；00.005 <；d / f12 <； Satisfy 16 十分に小型で、十分に広角で、十分に高解像度である内視鏡を実現するための撮像光学系を提供する。撮像光学系は、物体側から像側に配置された、負の屈折力を有する第１レンズと、正の屈折力を有する第２レンズと、絞りと、正の屈折力を有する第３レンズと、を備える。第１レンズの像側主点と該第２レンズの物体側主点との間の距離を｜ｄ｜、該第１レンズの像側主点が該第２レンズの物体側主点よりも像側にある場合に、両主点の間の符号付きの距離をｄ＝－｜ｄ｜とし、該第１レンズ及び該第２レンズの合成焦点距離をｆ１２として、d <； 00.005 <； d / f12 <； 16を満たす。
