PURPOSE: To prevent deterioration of the light receiving device thereby to enable accurate control of the luminance of the light source, by providing a water- cooled light receiving device for detecting the light signal for controlling quantity of light closer to the light source.

CONSTITUTION: Cooling piping 23 is fitted to a lamp house 22 including a light source 21 of a high pressure mercury lamp 20. One or a plurality of light receiving devices 24 are disposed between a correcting lens and the light source 21 but in the positions not interfering with the light beams from the light source for baking a shadow mask pattern on a glass panel. Since the light receiving device 24 receives the light emitted by the mercury lamp 20 through a spectral filter 27 and a diaphragm 28 and since the light receiving device 24 is cooled by means of water cooling piping 26, the same is free from damage due to heat and can bear long use. And, further, since the light receiving device 24 is disposed toward the correcting lens, it detects the light beams that are actually used for exposure, and therefore, accurate control of the luminance of the light source 21 can be made possible.