An endoscope illuminating optical system includes an illumination lens having a positive power. Illumination light rays parallel to each other emitted from an optical fiber bundle enter an incident surface of the illumination lens. The incident surface of the illumination lens is aspherical, and has a reference position vertical to the optical axis. The reference position is the most projecting portion on the incident surface. A shape of the incident surface is expressed by an equation of D=F(H). &ldquoH&rdquo is a height from the optical axis (0&nlEH&nlEHmax), and &ldquoD&rdquo is a depth from a reference plane. The reference plane is vertical to the optical axis. The reference position is on the reference plane. A first-order differential value of the function F(H) is more than 0, and a second-order differential value of the function F(H) is 0 at a particular height Hi (0
