Abstract: An alumina fiber aggregate that is formed of alumina short fibers and has been subjected to needling treatment, wherein the alumina short fibers have an average fiber diameter of 6.0 ?m or more and 10.0 ?m or less and a specific surface area of 0.2 m2/g or more and 1.0 m2/g or less, and a residual percentage (%) of high-temperature-cycle opened gap pressure of the alumina fiber aggregate is 45% or more. A value obtained by subtracting twice the standard error of a length-weighted geometric mean diameter of fiber diameters of the alumina short fibers from the length-weighted geometric mean diameter is 6.0 ?m or more. A proportion of alumina short fibers having a fiber diameter of more than 10.0 ?m is preferably 5.0% or less on a number basis.