An illumination apparatus of the present invention used for a microscope to guide illumination light to an objective lens attached to a revolving nosepiece comprises a light source for radiating the illumination light, an optical system for guiding the illumination light radiated from the light source, to the objective lens, an aperture diaphragm switching section provided in the optical system and having a plurality of aperture diaphragms and a light shielding portion, for switching the aperture diaphragm on the illumination light, linked with motion of the revolving nosepiece, when switching an observation method or the objective lens, and a control circuit for controlling operation of at least one of the aperture diaphragm switching section and the revolving nosepiece such that amount of light entering into the objective lens is prevented from undesirably increasing, when the observation method or the objective lens is switched.