An insulator for spark plug comprises an alumina-based sintered body comprising: Al2O3 as a main component; and at least one component (hereinafter referred to as “&bgr; component”) selected from the group consisting of Ca component, Sr component and Ba component, wherein the alumina-based sintered body comprises particles comprising a compound comprising the &bgr; component and Al component, the compound having a molar ratio of the Al component to the &bgr; component of 4.5 to 6.7 as calculated in terms of oxides thereof, and has a relative density of 90% or more.