Abstract: A spark plug exhibiting excellent withstand voltage characteristics and mechanical strength at high temperature, which are comparable to those at ambient temperature. The spark plug having an insulator formed of an alumina-based sintered material containing: an Al component, an Si component, a Group 2 element component, and a rare earth element component. The amounts of such components satisfy a condition (1): 0.25?oxide-reduced amount (mass %) of the Si component ?1.25, and a condition (2): 1.35?[(oxide-reduced amount of the Si component)/((oxide-reduced amount of the Group 2 element component)/(oxide-reduced amount of the rare earth element component))]?7.5. The alumina-based sintered material includes alumina grains, and a glass phase including a crystal phase that satisfies a condition (3) that the crystal phase includes at least one of an aluminate crystal phase and a silicate crystal phase, each having a grain size of 20 to 350 nm.

Abstract: A spark plug exhibiting excellent withstand voltage characteristics and mechanical strength at high temperature, which are comparable to those at ambient temperature. The spark plug having an insulator formed of an alumina-based sintered material containing: an Al component, an Si component, a Group 2 element component, and a rare earth element component. The amounts of such components satisfy a condition (1): 0.25? oxide-reduced amount (mass %) of the Si component ?1.25, and a condition (2): 1.35?[(oxide-reduced amount of the Si component)/((oxide-reduced amount of the Group 2 element component)/(oxide-reduced amount of the rare earth element component))]?7.5. The alumina-based sintered material includes alumina grains, and a glass phase including a crystal phase that satisfies a condition (3) that the crystal phase includes at least one of an aluminate crystal phase and a silicate crystal phase, each having a grain size of 20 to 350 nm.