PURPOSE: To detect the life of an exposure lamp, by detecting a current value required for making the quantity of light constant.

CONSTITUTION: When the quantity of light of an exposure lamp 4 is larger than a prescribed value, the base potential of a transistor TR 205 is higher than a ordinary value, and therefore, a capacitor 206 is charged gradually, and a time tA required for the output of a pulse voltage from a timer circuit 208 is relatively longer. The pulse voltage of the timer circuit 208 is outputted to the secondary winding of a pulse transformer 213, and a triplepole AC controlling element 214 is turned on. From this time t1 or t4, power is supplied from a power source 1 to the exposure lamp 4 through the first choke coil 6, and a capacitor 5 is charged simultaneously. When the voltage of the power source 1 becomes 0V, the element 214 is turned off, and the discharge of the capacitor 5 is started. That is, while the element 214 is turned on, the lamp 4 is lit to perform dimming. In this device, the lamp current is detected by the voltage between both ends of the choke coil 6 and is displayed on a meter 12 to discriminate the degradation of the exposure lamp quantitatively.