To provide an endoscope illumination optical system capable of improving insertability and reducing unevenness of light distribution. The illumination optical system 10 includes a transparent resin member 1 and an illumination member 2 that emits illumination light. The transparent resin member 1 includes a through hole 4 in which an imaging member 5 is disposed and an illumination member 2. The illumination light enters the transparent resin member 1 through the internal optical surface 7 and exits from the outer surface 6 of the transparent resin member 1 toward the object. The outer surface 6 of the transparent resin member 1 has a curved surface region 8 through which illumination light passes, and the curved surface region 8 has a boundary with the through-hole 4 formed in the region. The displacement from the boundary of each point in the region 8 occurs in a direction away from the object, and the shape of the curved region 8 in the cross section defined by the plane including the central axis of the through-hole 4 is within the curved region 8. The inclination angle at each point of the shape increases continuously and monotonously as the distance from the central axis increases. Condition (1), (2) are satisfied.挿入性の改善と配光ムラの低減を実現できる内視鏡照明光学系を提供すること。 照明光学系１０は、透明樹脂部材１と、照明光を出射する照明部材２と、を有し、透明樹脂部材１は、撮像部材５が配置される貫通孔４と、照明部材２が配置される非貫通孔３と、内部光学面７と、を有し、照明光は、内部光学面７を介して透明樹脂部材１に入射すると共に、透明樹脂部材１の外面６から物体に向かって出射し、透明樹脂部材１の外面６は、照明光が通過する曲面領域８を有し、曲面領域８は、領域内に形成された貫通孔４との境界を有し、曲面領域８では、曲面領域８の各点の境界からの変位が、物体から離れる方向に発生しており、貫通孔４の中心軸を含む平面で定義される断面内での曲面領域８の形状は、曲面領域８内の各点における傾斜角が、中心軸から離れるにつれて連続的かつ単調に増加する形状になっており、条件式（１）、（２）を満足する。
