An endoscope illuminating optical system to be disposed at a front end of an insertion portion of an endoscope includes a lens formed by a transparent-resin molding having a function of a front-end frame, which is configured to be disposed at a front-end side of the insertion portion, and a light guide which is disposed adjacent to the lens. A surface of the lens facing the light guide is a composite concave surface in which two axisymmetric concave surfaces are overlapped, and has a gourd-shaped effective cross-section. An effective range of a distal end surface of the light guide has a shape of an ellipsoid, or an oval, or a gourd, and has a long axis in a longitudinal direction of the gourd-shaped effective cross-section of the surface of the lens facing the light guide side, and a short axis in a direction perpendicular to the long axis. The cross-section of the composite concave surface includes an end surface of the light guide.