According to this invention, the positive and negative terminals of a high-tension capacitor are connected respectively to the anode and grid of the tripolar X-ray tube with a switching unit connected via a resistor beteween its grid and cathode. Based upon the current flown through the X-ray tube, the cut-off bias when the radiation being shut off and the grid bias for adjusting the focusing of the X-ray are established in the form of self-bias, therefor making the three-core high tenrion cable useable, while omiting the bias circuit for controlling the radiation and adjusting the focusing of the X-ray.