Alumina-based sintered body for spark plug and method of manufacturing the same, and spark plug and method of manufacturing the same

An alumina-based sintered body for a spark plug comprised of:

• alumina, a rare earth element oxide, and an oxide of the same metal as that contained in a sintering aid,
• wherein the alumina-based sintered body contains alumina in an amount of 90 mass % to 98 mass % and the rare earth element oxide in an amount of 0.2 mass % to 4 mass %, the balance being the oxide of the same metal as that contained in the sintering aid such that the alumina content, the rare earth element oxide content, and the content of the oxide of the same metal as that contained in the sintering aid is adjusted to 100 mass %, and
• a non-vitreous substance present in the grain boundary has an average grain size of 5 μm or less and a greatest grain size of 10 μm or less.