An illumination optical system which irradiates a sample surface with light through an illuminating lens includes: a light source; a condensing optical system receiving the light emitted from the light source; and a lens array optical system including a first lens array surface and a second lens array surface each formed by a plurality of lens elements. The first lens array surface has a conjugate relation with a back focal position of the illuminating lens. The second lens array surface is placed at a back focal position of the lens array optical system, has a conjugate relation with a pupil position of the illuminating lens, and the light source and the condensing optical system are arranged to form an image of the light source on the first lens array surface.