PURPOSE: To improve initial emission and life emission characteristics by dissolving scandium salt in a liquid medium, making this solution to permeate a cathode substrate, and heating-decomposing it thereafter.

CONSTITUTION: Scandium salt is dissolved in water, alcohol, and other liquid media and made to permeate a cathode substrate in form of the solution and then converted to scandium oxide by heating decomposition. As a result, the distribution of scandium oxide into the substrate is improved exceedingly and several excellent effects are obtained. In addition, since the sintering of the cathode substrate can be performed independently of addition of scandium, the high temperature sintering of the cathode substrate can be performed and the substrate with thermal resistance can be obtained.