PURPOSE: To maintain the electric insulation and the strength, and to improve the heat-resisting impact property by using a sintered body including Al2O3 as the main component and a specific wt.% of a specific additive.

CONSTITUTION: An insulator 3 is made by adding 0.5 to 5.0wt.% of BN or AlN powder to Al2O3 powder, and after a ball mill process is applied, sintering the mixture in an atmospheric furnace, and then applying an HIP process. An excellent heat conductivity of the AlN and BN contributes the improvement of heat conductivity of the insulator, and the electric insulation is also improved. And by adding AlN or BN, pores are liable to be generated, and the number of pores necessary to reduce the Young's modulus is secured by adding the above 0.5 to 5.0wt.%. And in the sintered body in which a sufficient number of pores are secured in such a way, the number of pores is not reduced by a warm pressure sintering, but the sizes of pores are made smaller to increase the strength, and this minuteness increases the heat conductivity. In such a way, the strength and the heat conductivity of the sintered body used as the insulator of a spark plug are increased. As a result, both the strength and the heat-resisting impact property are improved.