  Robinson and Wilczek suggested a new method of deriving Hawking radiation by
the consideration of anomalies. The basic idea of their approach is that the
flux of Hawking radiation is determined by anomaly cancellation conditions in
the Schwarzschild black hole (BH) background. Iso et al. extended the method to
a charged Reissner-Nordstroem BH and a rotating Kerr BH, and they showed that
the flux of Hawking radiation can also be determined by anomaly cancellation
conditions and regularity conditions of currents at the horizon. Their
formulation gives the correct Hawking flux for all the cases at infinity and
thus provides a new attractive method of understanding Hawking radiation. We
present some arguments clarifying for this derivation. We show that the Ward
identities and boundary conditions for covariant currents without referring to
the Wess-Zumino terms and the effective action are sufficient to derive Hawking
radiation. Our method, which does not use step functions, thus simplifies some
of the technical aspects of the original formulation.
