PURPOSE: To suppress the rapid oxidation at 800 to 900°C by adding a Cr oxide at a specific ratio to the above sintered body and heater consisting of silicon nitride ceramics contg. a conductive material at a specific ratio.

CONSTITUTION: The sintered body and heater of the conductive ceramics consist of 10 to 70vol.% conductive compd., 0.1 to 10wt.% Cr oxide and the balance silicon nitride ceramics. This sintered body and heater are improved in oxidation resistance at 800 to 900°C. The reason thereof lies conceivably in that the thick oxide film to be formed at ≥1000°C is not formed at a relatively low temp. of 800 to 900°C and the oxygen in the atm. diffuses through the grain boundary phase in the sintered body to progress the oxidation, but the diffusion of the oxygen in the grain boundaries is suppressed by the addition of the Cr oxide. The effect does not manifest if the amt. of the Cr oxide to be added is below 0.1%. The sinterability is low if the amt. exceeds 10%.