An illumination optical system for an endoscope includes two light guides disposed in an insertion tube in a first direction to sandwich a center of the insertion tube therebetween; an observation window on a tip end face of the insertion tube; two concave lens parts having negative powers sandwiching the observation window at positions facing end faces of two light guides on the tip end face of the insertion tube. The end face of each of the two light guides has a smaller width in the first direction than a width in a second direction perpendicular to the first direction; each of the two concave lens parts has a larger negative power in the first direction than a negative power thereof in the second direction; and of illumination light which has propagated through each of the two concave lens parts after being emitted from each of the two light guides.