Spark plug

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To provide a spark plug having an insulator which has excellent corrosion resistance and which exhibits, at high temperature, excellent withstand voltage characteristics and high mechanical strength; i.e., small percent drops in withstand voltage characteristics and mechanical strength when the temperature rises further from a high temperature. The spark plug of the present invention has an insulator formed of an alumina-based sintered material which contains an Si component and a rare earth element component in such amounts that satisfy the following conditions (1) and (2) : (1) 0.05 ‰¤ oxide-reduced amount of the Si component (mass%) ‰¤ 0.45; and (2) 2.5 ‰¤ [(oxide-reduced amount of the rare earth element component)/(oxide-reduced amount of the Si component)] ‰¤4.5, and which has alumina grains and an alumina grain boundary phase including a complex oxide crystal phase satisfying the following condition (3) : (3) the complex oxide crystal phase contains at least two components selected from among an Al component, an Si component, a Group 2 element component, and a rare earth element component.