A microscopic illumination apparatus to be applied to a microscope selectively using plural types of objective lenses having different magnifications comprises a light source, a collector lens for making a light flux emitted from the light source a substantially parallel light flux, and at least two fly-eye lenses disposed side by side along the optical axis at the back of the collector lens. A space between the two fly-eye lenses is variable in accordance with an objective lens to be used in the microscope.