There is provided an endoscope illuminating optical system which is capable of realizing an improvement in the insertability and a reduction of the uneven light distribution. The endoscope illuminating optical system includes a transparent resin member, and a lighting member which outputs illumination light. The transparent resin member has a through hole in which an image sensor is disposed, a non-through hole in which the lighting member is disposed, and an internal optical surface. The illumination light is incident on the transparent resin member via the internal optical surface, and is emerged toward an object from an outer surface of the transparent resin member. The outer surface of the transparent resin member has a curved-surface area through which the illumination light passes, and the curved-surface area has a boundary with the through hole which is formed in the curved-surface area. In the curved-surface area, a displacement of each point of the curved-surface area from the boundary occurs in a direction away from the object. A shape of the curved-surface area in a cross-section defined by a plane including a central axis of the through hole is a shape such that an angle of inclination at each point in the curved-surface area increases continuously and monotonously as drawn away from the central axis, and conditional expressions (1) and (2) are satisfied.
