Abstract: An F-kV and an F-mA output from tube-voltage adjuster and tube-current adjuster, respectively, are supplied to exposure-rate controller. Controller compares an input entrance exposure rate determined by the F-kV and the F-mA with a preset exposure rate set in view of a patient's safety. When the former is equal to or less than the latter, controller supplies the input F-kV and F-mA to X-ray tube. When the former exceeds the latter, controller supplies an F-kV and an F-mA corresponding to the preset exposure rate to X-ray tube and a limit signal to auto-iris controller. X-rays output from X-ray tube are transmitted through patient to form an X-ray image. The X-ray image is converted into an optical image by image intensifier tube. After the light quantity of the optical image is reduced by auto-iris, the optical image is picked up by TV camera. TV camera outputs an image signal to TV monitor and brightness detector.