PURPOSE: To conveniently and inexpensively produce an alumina fiber excellent in fiber shape retaining property and useful as a reinforcing material, adsorbent, fire resistant additive and fibrous material precursor by subjecting a fibrous aluminum salt to oxidation treatment with a chloric acid aq. soln.

CONSTITUTION: The fibrous hydrated alumina having 20-35wt.% Al content is obtained by subjecting the fibrous aluminum salt to the oxidation treatment with the chloric acid aq. soln. In a production of this fibrous hydrated alumina, for example, the fibrous aluminum salt such as fibrous aluminum sulfate and fibrous aluminum carbonate is subjected to the oxidation treatment with the chloric acid aq. soln. preferably. This fibrous hydrated alumina is a new one having 20-35wt.% aluminum content and rich in adsorptivity of other substance and useful as fibrous aluminum based multiple oxides.