PURPOSE: To obtain a high-level dimming function suitable to an automated microscope by dimming a light source automatically corresponding to a detected objective.

CONSTITUTION: A CPU reads the magnification and the kind of objective 8 out of a storage part 35 by a signal from an objective switching driving part 26 to select a proper condenser lens 6 on the basis of those data, and also sets proper diameters of an aperture stop 5 and a filled stop 4 at the same time. Further, the CPU32 selects a combination of the quantity of light of the light source and an ND filter 3 on the basis of the objective data to set specific standard brigtness. Then, when the power source is turned on, the illumination light is set automatically to a standard state.