Spark plug

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A spark plug having an insulator formed of an alumina-based sintered material which contains an Si component and a rare earth element component that satisfy the following conditions (1) and (2) : (1) an oxide-reduced amount of the Si component (mass %) is greater than or equal to 0.05 and less than or equal to 0.45; and (2) a ratio of an oxide-reduced amount of the rare earth element component to an oxide-reduced amount of the Si component is greater than or equal to 2.5 and less than or equal to 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.