PROBLEM TO BE SOLVED: To provide an illumination optical system of an endoscope capable of reliably holding and protecting a light guide fiber and distributing light from a light source with no waste.SOLUTION: An illumination optical system of an endoscope includes a light guide fiber disposed in an insertion part of an endoscope for guiding illumination light, and a cover member covering an emission end surface of the light guide fiber and disposed at the leading end of the insertion part for distributing the light emitted from the light guide fiber to a field of view of an observation optical system. The cover member includes an annular core made of transparent glass surrounding the observation optical system, and a reflection surface formed at least on an inner peripheral surface side of the core for totally reflecting the light.
