There is provided an endoscope illuminating optical system in which a favorable light-distribution can be achieved even in a case of a wide in-water angle of view, and which is suitable for mounting in a medical endoscope having a thin diameter. An endoscope illuminating optical system at a front end of an insertion portion of endoscope includes a lens L which is disposed at a front-end side of the insertion portion, and a light guide 2 which is disposed adjacent to the lens, wherein a surface of a lens L facing the light guide 2 is a composite concave surface 12 in which two axisymmetric concave surfaces are overlapped, and has a gourd-shaped effective cross-section, and an effective range of the light guide 2 has a shape of a non-circular end surface having a long axis in a longitudinal direction of the gourd-shaped effective cross-section on a side of the surface of the lens L facing the light guide side, and a short axis in a direction perpendicular to the long axis.
