PROBLEM TO BE SOLVED: To reduce the effective diameter of a light distribution member and to provide proper light distribution over the entire visual field range in an illumination optical system of an endoscope in which a pair of light guides are disposed across an observation optical system and the light outgoing from each of the light guides is diffused through the respective light distribution members.SOLUTION: Each light distribution member has a convex lens part which is oppositely located to each emission end surface of a pair of light guides and has positive power to once condense a light flux emitted from each of the light guides in each of the light distribution members. The convex lens part of each of the light distribution members has light distribution characteristics that cause the light flux emitted from each of the pair of light guides to be emitted from the emission surface of the light distribution member at a light distribution angle which is greater than the view angle of an observation optical system in the vicinity of the optical axis of the observation optical system and which substantially matches the view angle in the peripheral edge side in the visual field range of the observation optical system.
