Abstract: This invention relates to a sintered body of conductive sialon and a heater element made of this sintered body. They are relieved from damage by partial expansion without any adverse effect on mechanical strength and electrical properties. According to this invention, the thickness of the oxidation layer in the sintered bodies is controlled. Thus the sintered body of sialon pertaining to this invention is formed by sintering a mixture composed of a base mixture containing Si.sub.3 N.sub.4 (major constituent), a small amount of AlN (polytype), and less than 5 wt % of Y.sub.2 O.sub.3, and a small amount of Al.sub.2 O.sub.3 and 23-70 vol % (based on the base mixture) of at least one kind of Ti carbide, Ti nitride, and Ti carbonitride.

Abstract: A glow plug for diesel engines includes a ceramic heater with a U-shaped heating portion and a pair of leads integrally formed with the heating portion and extending backwards from both ends of the heating portion. The heater is supported at one end thereof by a hollow holder in a cantilevered state with one end thereof protruding towards the outside. At least the outside surfaces of the leads are joined to, and held by, the inside of the holder via an insulating layer, and at least the rear ends of the other of the leads are connected via a metallic lead wire to an external connection terminal supported in an insulated state by the rear end of the holder. The ceramic heater is integrally formed from an electrically resistive sintered sialon produced by adding 20-70 vol. % Ti nitride or carbonitride as an electric conductivity donor to .beta.-type sialon or a .alpha./.beta.-type sialon.