Abstract: An automatic illumination intensity control apparatus for a discharge lamp includes a chopper circuit for stepping up D.C. voltage obtained by rectifying commercial A.C. voltage, an invertor circuit for converting D.C. voltage output from the chopper circuit into an A.C. voltage with a higher frequency to drive the discharge lamp, a brightness level sensing portion for sensing the brightness level of the area to be illuminated, a brightness level setting portion for setting brightness level of the area to be illuminated, and a power cut-off portion for stopping the operation of the invertor circuit when the sensed brightness level is greater than the set brightness level. The control apparatus may further include an illumination intensity adjusting portion for varying voltage output from the chopper circuit in proportion to the difference between the set brightness and sensed brightness when the sensed brightness is less than the set brightness.