The invention provides lenses at low costs, which form an illumination optical system for endoscopes that has high efficiency and achieves improved light distribution. The illumination optical system for endoscopes is used in opposition to a light beam exit end 1 of a transmission means F for light out of a light source, and comprises, as viewed from the light beam exit end 1, a lens group 10 having positive power and an optical member 20 subsequent to thereto which has a spherical surface 20a that has a radius of curvature R and lens action, and satisfies the following condition: 1.48≦S/πR2≦4   (1) where R is the radius of curvature of the spherical surface, and S is a surface area of the spherical surface.