PURPOSE: To prevent the breakage of wire due to the heater elements becoming fragile by causing leg portions thereof to be coated with a sintered material layer composed of a mixture of a high-melting point metal powder and a metal powder having a melting point lower than this high-melting point.

CONSTITUTION: A cathode pellet 4 is soldered to a sleeve 2. Inside the sleeve 2, heater elements 1 are buried in an alumina powder sintered material 5. Leg portions 8 of the heater element exposed from the sleeve are each formed with a platinum coating layer 6. Over the outer periphery of this layer 6 a sintered material layer 7 consisting of a mixture of tungsten/nickel powder is formed. The layer 6 is prevented from becoming fragile due to the thermal diffusion, into the leg portion 8, of the nickel component contained in the layer 7.