An accurate calculation method for Fermi surfaces is presented in order to study the electronic doping effect to semiconductor alpha-boron. Given the eigenvalues of energy at a limited set of k points, the present interpolation method retains the original accuracy of the energy eigenvalues, which are provided by a band calculation program. A significant contribution of the present method is to eliminate spurious components in the interpolation which are present in other methods. This is achieved by removal of redundancy in the basis functions. An application is given for doped-semiconductor alpha-boron, which has recently attracted a growing interest in connection to superconductivity research.