PURPOSE: To obtain large X-ray output with the use of a glass envelope, by attaching a shield plate made of a heat-resistant insulation material to a cathode so as to shield the focusing electrode of the cathode and the target of an anode from the glass envelope.

CONSTITUTION: A shield plate 7 is attached and fixed to a cathode 2a. The shield plate 7 is made of a heat-resistant insulation material such as quartz or porcelain, and disposed between a glass envelope 1 and a focusing electrode 3, target 5 and so on. The size of the shield plate 7 is determined so that the plate covers at least a region of the inner surface of the glass envelope 1 on which electrons dispersed from the focus 6 can impinge. When the X-ray absorption of the shield plate 7 is large, a transmittable hole 8 is provided at least in the area where the X-ray cone passes through. In such a manner, the heat radiated from the target and the electrons dispersed from the focus during generation of X-rays do not enter directly into the envelope but penetrate in the shield plate. Consequently, even if the X-ray output is increased, the gas is effectively prevented from being emitted from the envelope.