This invention provides an X-ray tube apparatus which can output X-rays of a dose suitable for radioscopy for a long time. In the apparatus, small focus filaments are provided on respective sides of a large focus filament, such that they have almost equal distances from the center of the large focus filament, and the inclination angles of converging electrodes surrounding the respective small focus filaments with respect to a cathode main body are set to almost equal angles within a range of 20 to 40.degree..