Insulator and method of manufacturing an insulator

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The present invention relates to an insulator comprising an alumina-based sintered body containing at least one rare-earth element, said alumina-based sintered body having a theoretical density ratio of 95% or more and containing 0.01 to 18 wt.% of said rare-earth element as reduced to oxides, wherein the alumina-based sintered body further contains 0.05 wt.% or less of sodium as reduced to oxides, and wherein the alumina-based sintered body further contains silicon, calcium, or magnesium, wherein, provided that the silicon content is taken as S (unit : wt.%) as reduced to oxides, the calcium content is taken as C (unit : wt.%) as reduced to oxides and the magnesium content is taken as M (unit : wt.%) as reduced to oxides, the expression S / (S+C+M) ≥ 0.7 is satisfied, and to a method of manufacturing an insulator.