A method for preparing tantalum oxide and/or niobium oxide, characterized in that it comprises adding a basic aqueous solution to an aqueous solution of tantalum fluoride salt and/or niobium fluoride salt, to form tantalum hydroxide and/or niobium hydroxide, and then firing said tantalum hydroxide and/or niobium hydroxide. The above method allows the preparation of the tantalum oxide and/or niobium oxide in the state of a needle crystal or columnar crystal, not of a spherical crystal or an assemblage form.