hours and shown in longitudinal section in fig2 the heating element 2 and the sleeve 15 of which both contain al 2 o 3 are fused with each other when they are fired then molybdenum is metallized on the surfaces of the grooves 202 and 203 and nickel is plated on the surfaces of the molybdenum layers a ceramic insulator 16 composed of alumina is inserted into the sleeve 15 then the terminals 41 and 42 are inserted into the sleeve 15 through the grooves 161 and 162 of the insulator 16 and the grooves 202 and 203 of the heating element 2 a silver braze is applied to the ends of the terminals 41 and 42 projecting from the sleeve 15 the sleeve 15 is inserted into the housing 3 and one of the terminals 41 and 42 is brought into contact with the inner surface of the housing 3 while the other terminal is engaged in a groove formed in the central electrode 4 but not shown then one end 302 of the housing 3 is annually deformed into the recess 151 of the sleeve 15 as shown in fig2 the whole assembly is placed in an electric furnace so that the terminals 41 and 42 may be brazed at one end to the grooves 202 and 203 of the electrode portions 22 and 23 of the heating element 2 while the other ends of the terminals 41 and 42 are brazed to the central electrode 4 and the inner surface of.