An X-ray diagnostic apparatus and an auxiliary device, such as an X-ray photographing device which requires a fixedly predetermined exposure time, and which contains a control circuit for the X-ray dosage power or output with reference to a patient; a dosage output measuring arrangement; a comparator element connected to the latter for comparing the actual value of the X-ray dosage output with a reference value, which effects optimum film darkening for each particular exposure time; and adjusting means controlled by the output signal of the comparator element for adjustment of the X-ray tube voltage for equating the actual value of the dosage output with the reference value.