PURPOSE: To obtain an x-ray photo with more stable picture quality by operating a photo timer within the upper and lower bound time of the x-ray photographing time set by x-ray penetration conditions.

CONSTITUTION: When the output of a preamplifier 12 that is proportional to the amount of lighting reaches the preset reference voltage, the signal that is proportional to the preset x-ray penetration tube voltage is generated from an x-ray penetrating tube voltage signal generator circuit 20. Then an x-ray pickup tube voltage preset circuit 27 presets the x-ray penetration tube voltage based on the optimum x-ray penetration tube voltage and then the upper and lower bounds of the x-ray photographing time through an x-ray photographing time upper bound preset circuit 21 and a lower bound circuit 22. The x-ray irradiation is controlled at the photographing time preset to the optimum exposure conditions of photo film by controlling a back up timer circuit 25 within such a preset x-ray photographing range.