A lighting optical device is disposed in a distal portion of an endoscope, and illuminates a target with light transmitted through a light guide. An optical element has an incident surface facing the light guide, an outer circumferential portion having light reflection function, and a convex exit surface. In a lens mounting hole formed in the distal portion, a lens is housed in front of and at a predetermined distance away from the optical element fixed in a lens barrel. An outer diameter of the lens is greater than that of the optical element. To allow light rays, coming out of a peripheral portion of the exit surface of the optical element and traveling parallel with an optical axis, to be incident on the lens, a diameter of a space between the optical element and the lens is greater than or equal to the outer diameter of the optical element.
