For the purpose of performing illumination with which vignetting of illumination light caused by a structure is alleviated, thus making the difference between a bright part and a dark part on an object less noticeable, an illumination device according to the present invention is provided with: a light-emitting part that has an emission end from which illumination light is emitted; a diffusion optical member that is disposed around a predetermined axis in the circumferential direction, that is provided with an incident end disposed so as to be opposed to the emission end, that guides the illumination light entering from the incident end while diffusing the illumination light, and that emits the illumination light from a surface thereof; and a reflective layer that is disposed adjacent to a radially inward surface of the diffusion optical member and that reflects the illumination light radially outward, wherein an angle, around the axis, of an emission region for the illumination light in the diffusion optical member is reduced as the distance from the incident end is increased in the axial direction.