PURPOSE: To preclude a defect in the fixation of printed matter due to stains on plate glass by providing a light quantity detector to a heat sink part below a xenon lamp and checking its output.

CONSTITUTION: The heat sink 16 below the xenon lamp 13 is provided with the photodetection part of the light quantity detector 17 and light is guided from the lamp by a glass fiber 19 and inputted to the light quantity detector 17. The xenon lamp 13 is turned on to illuminate when there is no form 10 between the xenon lamp 13 and heat sink 16, its flash is received by the photodetection part 18, guided by the glass fiber 19, and inputted to the light quantity detector 17, whose output is confirmed and compared with the previously set output of the light quantity of the light quantity detector 17. Consequently, the defect in fixation due to stains on the glass 14 provided in front of the xenon lamp 13 can be precluded.