An object of the invention is to provide an alumina fiber aggregate which, when used as a filler in various materials such as, e. g. , resins, coating materials, metals, and ceramics, enables the mixtures to have reduced viscosity and thereby heightens the productivity of products containing alumina fibers, and which enables cured thermosetting resins to have a sufficiently heightened thermal conductivity.
This alumina fiber aggregate, when examined by the electron microscope method, gives a frequency distribution regarding number of fibers, wherein the number of fibers having a fiber length of 50-200 µm is 70% or more (including 100%), the number of fibers having a fiber length smaller than 50 µm is 30% or less (including 0), the number of fibers of 50-100 µm is 30-50%, the number of fibers longer than 100 µm but not longer than 200 µm is 30-50%, and the number of fibers longer than 200 µm is 20% or less (including 0). The fibrous mass comprises 70-100% by mass of Al 2 O 3 and 0-30% by mass of SiO 2 .