An endoscope illumination optical system includes a surface light source, a positive first lens element, a positive second lens element, in this order from the surface light source.The endoscope illumination optical system satisfies the following conditions:0.40<；f/D<；0.53 (1)0.40<；0.5·D·θ<；0.56 (2)whereinf: the combined focal length of the positive first lens element and the positive second lens element；D: the maximum diameter of the surface light source；R1: the radius of curvature of a first surface, of the positive first lens element, facing toward the surface light source；R2: the radius of curvature of a second surface, of the positive first lens element, facing toward a surface to be illuminated；n1: the refractive index of the positive first lens element；d1: the thickness of the positive first lens element； andθ designates an angle defined as (1+0.85d1/R2)(1−1/n1)/R1−1.05/R2.
