PURPOSE: To obtain a long life impregnated-type cathode secular-change of whose electron emission characteristic is small, by a using a tungsten layer of large porosity rate as the lower layer and by using tungsten layer of small porosity rate as the surface layer of the captioned cathode.

CONSTITUTION: The figure shows as impregnated-type cathode 10 designed by the present invention, a holding cylinder 3 made of molybdenum, and a cathode structure 11 of a heater 4 buried in sintered alumina 5. The captioned cathode 10 is a laminated structure consisting of two layers, that is, the surface layer 1 with the porosity rate of 20% and the lower layer 2 with the porosity rate of 40%. In the captioned cathode 10, the evaporation amount of Ba can be suppressed and the necessary amount of Ba can be supplied by reducing the porosity rate of the surface layer 1, so that the consumption-time of Ba can be prolonged. On the other hand, by enlarging the porosity rate of the lower layer 2, the supply amount of Ba from the inside of the vacant-hole to the surface layer can be increased, the depreciation of Ba-covering degree necessary for electron emission can be suppressed, and the secular-change of current can be minimized.