PURPOSE: To prevent the deterioration and the like of temperature responsiveness, and to obtain the stabilized quantity of light by a method wherein temperature is almost uniformly maintained by controlling the exposure time and the non-exposure time.

CONSTITUTION: On the flush type mercury lamp 1, which is used as an exposure light source, power is fed by an ignition device 2 which can be switched to two different making powers for an idling lighting and a flush-up lighting in synchronization with the shutter 6 provided in a projection optical system, and the lamp 1 is lighted. An air cooling fan 4 is arranged above the mercury lamp 1, and the mercury lamp 1 is forcedly cooled by the blast quantity controlled by a control unit 5. A temperature sensor 3 is attached to the mercury lamp 1, the temperature of the lamp tube part is detected by the temperature sensor 3, and it is given to the control unit as the control information. As a result, the temperature responsiveness and the like when operation is moved to exposing one can be improved, and the desired stable quantity of light can be obtained from the light source when moving to the exposure operation.