PURPOSE: To prevent a higher harmonic microwave from leaking out of a light source device used for a printing device for printing an image on an original film on a photosensitive material.

CONSTITUTION: As to a cavity 3 in which an electrodeless discharge lamp 4 is installed, the upper part of the cavity 3 is provided with an opening part 7. A shutter 11a can be moved on the opening part 7 so as to shut the opening part 7 in an initial transient state where the higher harmonic microwave leaks in the cavity 3. Printing is performed with the shutter 11a kept on at the time of normally emitting the light. Thus, the microwave can be prevented from leaking without reducing the light quantity which is obtained at the time of normally emitting the light.