Abstract: A sensor-controlled illumination apparatus is provided whose response to light is basically made at a small time constant. This is applicable both when brightening illumination and when darkening the illumination. However, in the case of the darkening, a darkening speed is limited when a decrease in luminance per unit time exceeds a limit value. The above-described method makes it easy to successfully adapt the illumination to given necessity on one hand, and on the other hand, allows people in an illuminated area to feel less uncomfortable with the sequential adjustment of the illumination, or in an ideal case, to be unaware of the adjustment.