An endoscope illuminating optical system having a gradient index lens capable of illuminating by a sufficient light amount and by a wider angle is provided. The endoscope illuminating optical system is constituted to illuminate an observed region by light guided by a light guide 1, by way of a gradient index lens 2, and the gradient index lens 2 satisfies Condition Equations (2) and (3) below: (AR).sup.2.ltoreq.0.6 (2) 0.5k+0.35.ltoreq.AL/.pi..ltoreq.0.5k+0.65 (3) where, R designates an effective radius of the gradient index lens, L designates a length in an optical axis X direction of the gradient index lens, A designates a constant coefficient, k designates a nonnegative integer.