Illumination optical system and endoscope

An illumination optical system disposed on an emission side of a light guide of an endoscope,

• wherein in a case where a surface of the illumination optical system closest to a light guide side is defined as a first surface, and a surface of the illumination optical system closest to an irradiation target side is defined as a second surface,
• an outer peripheral surface of the illumination optical system from the first surface to the second surface has a light reflecting surface on a first surface side and has a light absorbing surface on a second surface side,
• a stepped surface that is a surface perpendicular to an optical axis of the illumination optical system and that constitutes a step at which an outer diameter dimension changes is formed at a boundary between the light reflecting surface and the light absorbing surface,
• the first surface has a plurality of first concave surface portions that have the same curvature radius and are provided at a small-diameter portion having a smaller diameter than a diameter of an outermost diameter portion at which an outer diameter of the illumination optical system is largest, and
• in a case where a radius of the small-diameter portion is defined as C,
• a radius of the outermost diameter portion is defined as H,
• a distance from a position on the first surface closest to the light guide side to the stepped surface in a direction of the optical axis is defined as D,
• a distance from an intersection point between the second surface and the optical axis to the stepped surface in the direction of the optical axis is defined as E, and
• the curvature radius of the first concave surface portion is defined as R1,
• Conditional Expressions (1), (2), and (3) represented by
• 0.4<C/H<0.95 (1),
• 0.45<D/(D+E)<0.95 (2), and
• 0.05<R1/H<0.5 (3) are satisfied.