PURPOSE: To secure such a columned beam as being stable for a long period of time, by shielding an electron of an oxide cathode using Zr and the like as a reducing agent, from the peripheral part of an oxide layer where an intermediate product is formed on the interface whereby electronic radioactivity is liable to deteriorate.

CONSTITUTION: A shielding plate 24 is installed in order to let no electron pass through from the peripheral part of an oxide layer 22 sticking to an Ni base body metal 21 containing, for example, Zr by 0.1wt%. The shield of this electron to be radiated is somewhat larger than the 30μ width of an intermediate product 23a in order to make sure of the effect, and covers up to the inside of 50μ from the peripheral part of the oxide layer. Furthermore, the cap-like shielding plate 24 is made up of, for example, an Ni plate of 0.1mm in thickness and set up in a way of inserting its cylindrical part into the base body metal after applying a carbonate layer onto the base body metal to be turned to the oxide layer 22 upon decomposition by heating. With this, the cause to make an electron tube result in a short life so far is eliminated, so that such a columned beam as being stable for a long period of time can be secured.