PURPOSE: To prolong the service life of a linear thermal cathode by coating a linear heat-resistant metal with an electron emitting material of barium- containing alkaline earth metal oxide containing a scandium oxide in a specified content range.

CONSTITUTION: A linear heat-resistant metal is coated with an electron emitting material which is at least barium-containing alkaline earth metal oxide and contains 20-40wt.% of scandium oxide. Due to that the electron emitting material layer formed on a metal substrate such as tungsten contains 20-40wt.% of scandium oxide, an interlayer of mainly Ba3WO6, etc., formed in the interface of the tungsten substrate and the electron emitting material is decomposed and the decomposition function against Ba3WO6 is sufficiently maintained even when the operational temperature increases and the amount of the Ba3WO6 increases. In this way, the accumulation of the interlayer is suppressed and lowering the service life is avoided.