Abstract: An X-ray generating device comprises a focal electrode (10) placed adjacent to the cathode (9) and provided with a focal aperture (10a) aligning with the cathode. A positive voltage is applied to the focal electrode relative to the cathode so that equipotential lines around the focal aperture may bulge toward the X-ray target (2). Thereby, the part of the electron beam having a large diverging angle is captured by the focal electrode, and only the part of the electron beam having a uniform diverging angle is allowed to pass through the focal aperture so that the electron beam can be favorably micro focused upon the X-ray target and can be given with a high intensity.