PURPOSE: To prevent the output data of an AD converter from overflowing by automatically compensating tube current depending upon the types of subjects.

CONSTITUTION: When the quantity of X-ray absorption is reduced depending upon the types of subjects 3, the output data of an AD converter 5 increases. When this value generates overflow, a pulse generator 10 operates and a pulse output S4 is recognized by a CPU7 through an interface circuit 6. A gate circuit 15 operates and a counter 16 is decremented. By this output of this counter 16 changes that of a decoder 17, the lower stage relay is selected in the relay circuit 18, and the tube current value is compensated so as to decrease.