PURPOSE: To control X-ray exposure to a minimum and to decrease the heat capacity of an X-ray tube, by disposing a projector and a light detector which irradiate the light to a body to be tested in the vicinity of the X-ray tube and the X-ray detector, and to control the X-ray exposure by detecting signal of the light detector in the case of linear scanning.

CONSTITUTION: When an X-ray tube 1 and an X-ray detector 2 are subjected to linear scanning from position A to position B by driving a pedestal 3, a light beam irradiated from a projector 4a is broken by a body P to be tested; the detecting signal at the interruption of light is output from a light detector 5a to a signal generating circuit SG for the period of exposure. At the same time, the detecting signal from a position detector SP of the pedestal 3 is input into a circuit SG; and with the timing of right end the body P to be tested, the X-ray exposure is started by a power source switching circuit SC. Next, the light beam of a projector 4b is also broken, and consequently, the interruption of the projector 4a side is recovered, these signals are detected by the circuit SG, thus the X-ray exposure is stopped with the timing of the left end of the body P to be tested. Consequently, the X-ray exposure can be controlled to necessary minimum and also the heat capacity of the X-ray tube can be decreased.