PURPOSE: To enable a stable focus with both clear outline and homogeneous electron density to be obtained, by forming a thin beltlike material made of a heat-proof metal and haing homogeneous width and thickness into a U-shape, and using the center of the U-shape for thermionic emission, and the ends of the U-shape for the support of the U-shape structured body.

CONSTITUTION: Thin beltlike material which is made of a heat-proof metal and has a homogeneous width and thickness is formed into a U-shape, which consists of a thermion emitting part 6 in its center, and supported fixed parts 7 at its ends. By the means mentioned above, the material loss can be decreased, and the work can be performed easily. In addition, for the purpose of preventing any sag and warping of the part 6 which might be caused due to high temperature, snap plates 8 used as a spring are provided on both sides of each of the parts 7, and each end of the U-form material and the snap plates 8 are supported by and fixed to an anchor 9. Consequently, a structured body for an X-ray tube having a rectangular thermion emitting surface with a good focus, can be made in a high material yield.