PURPOSE: To determine a tube voltage required when photographing for an adjusted pulse period and to decrease exposed dose, by providing that a tube voltage when a dose rate really measured by X-rays generated in a trapezoidal high voltage pulse and a setting dose coincide each other is to be a photographic tube's voltage.

CONSTITUTION: A detector 3 measures a dose rate, by irradiating X-rays in a trapezoidal 1 pulse of high voltage generated by a trapezoidal pulse generator 13 to a subject body. Inputting an output of the detector 3 into a comparator 6 and comparing it with the dose rate previously setted up by a setting device 5, when both of them coincide each other, their output of a tube voltage measuring device 9 through a monostable multivibrator 7 and after being converted by an A/D converter 10, it is stored in a latch circuit 11. An X-ray tube 1 therefore is interrupted. When X- ray photographing is made, a tube voltage value of the latch circuit 11 shall be applied to a high voltage generator 16 through a tube voltage control circuit by switching a switch 14. Due to this, it can be achieved to measure a tube voltage necessary for photographing in exposed X-ray of 1 pulse and not to give excess dose to the subject body.