PURPOSE: To provide cathode structure where a focus position and focus overlap are nearly changed before and after a filament of an X-ray tube is heated.

CONSTITUTION: A grove 9 having a width of about 0.2-0.3mm is formed on the center of a focusing body 1, thus weakening rigidity in the center of the focusing body. Accordingly, thermal expansion influence of the focusing body generated by heating a filament is concentrated on the groove 9 in the center, thereby restraining a change in angle of the focusing body. Consequently, a focus position and coincidence thereof are nearly unchanged before and after the filament is heated, resulting in remarkable reduction of artifact on a CT image.