A microscope comprising: an observation optical unit that includes a zoom mirror incorporating a zoom optical system having variable zoom power and an objective lens optically connected to the zoom optical system, the observation optical unit receiving observation light emitted from an observation sample; a microscope main body (4,11) that holds the observation optical unit such that the observation optical unit is freely attachable and detachable in a direction perpendicular to an optical axis of the objective lens; a focusing unit that includes a focusing mechanism to focus the observation optical unit on the observation sample; a light source unit that emits illuminating light to illuminate the observation sample; and an illuminatory optical system (4a,12,13,14) that includes a reflecting optical element; a post member that is disposed upright in a direction of focusing of the observation optical unit by the focusing unit, the post member supporting the focusing unit such that the focusing unit is freely movable in the direction of focusing, wherein the illuminatory optical system (4a,12,13,14) is partially disposed in the focusing unit so as to be parallel to the post member, and the reflecting optical element of the illuminatory optical system (4a,12,13,14) is disposed outside the focusing unit and out of the optical path of the zoom optical system but, in a state when the observation optical unit is attached to the main body (4, 11), inserted in the zoom mirror, and wherein the illuminatory optical system (4a,12,13,14) is adapted to illuminate the observation sample with the illuminating light via the reflecting optical element and the objective lens. an observation optical unit that includes a zoom mirror incorporating a zoom optical system having variable zoom power and an objective lens optically connected to the zoom optical system, the observation optical unit receiving observation light emitted from an observation sample; a microscope main body (4,11) that holds the observation optical unit such that the observation optical unit is freely attachable and detachable in a direction perpendicular to an optical axis of the objective lens; a focusing unit that includes a focusing mechanism to focus the observation optical unit on the observation sample; a light source unit that emits illuminating light to illuminate the observation sample; and an illuminatory optical system (4a,12,13,14) that includes a reflecting optical element; a post member that is disposed upright in a direction of focusing of the observation optical unit by the focusing unit, the post member supporting the focusing unit such that the focusing unit is freely movable in the direction of focusing, wherein the illuminatory optical system (4a,12,13,14) is partially disposed in the focusing unit so as to be parallel to the post member, and the reflecting optical element of the illuminatory optical system (4a,12,13,14) is disposed outside the focusing unit and out of the optical path of the zoom optical system but, in a state when the observation optical unit is attached to the main body (4, 11), inserted in the zoom mirror, and wherein the illuminatory optical system (4a,12,13,14) is adapted to illuminate the observation sample with the illuminating light via the reflecting optical element and the objective lens.