PURPOSE: To prevent the exfoliation of alumina insulation layer due to cracks generated in which or the leakage current between the heater and the cathode of an electron tube by using alumina dispersed with metalic molybdenum particles as the alumina insulating layer.

CONSTITUTION: In the alumina layer metalic molybdenum particles are dispersed. Thereby as the insulating strength of the insulating layer is possibly degraded, so an insulating layer consisting of only alumina 2 is formed on the tungsten core wire 1, and on the said insulating layer an additional insulating layer consisting of alumina 2 dispersed with metalic molybdenum 3 is provided. Or it is also effective to provide an insulating layer consisting of only alumina 2 on the insulating layer consisting of alumina dispersed with metalic molybdenum particles 3 formed on the tungsten core wire 1. The dispersion of metalic molybdenum particles is desirable to be 5W20vol.% because when it is lower than 5vol.% no effect is obtained and when it is higher than 20vol.% the dispersion amount of the particles and the insulating strength of the layer are degraded significantly.