A microscope including an illumination optical system having an objective lens and a fly-eye lens, where: the fly-eye lens having a plurality of lens elements; in the illumination optical system, images of the plurality of lens elements are projected on a pupil of the objective lens; and the number (n 1 ) of the images of the lens elements projected inside the pupil of the objective lens is more than the number (n 2 ) of the images of the lens elements projected on an outer perimeter of the pupil of the objective lens. In the above observation optical system, a first objective lens having a first pupil diameter and a second objective lens having a pupil diameter larger than the first pupil diameter are switchable, and a number n 1 may be greater than a number n 2 with respect to the first objective lens.