  In this Letter we have derived the gravitational anomaly leading to the
Hawking radiation from a fundamentally different perspective: it emerges due to
the {\it{complimentary}} roles played by tunneling and (gravitational) anomaly.
We have used the analogy of an early idea \cite{niel1} of visualizing chiral
gauge anomaly as an effect of {\it{spectral flow}} of the energy levels, from
the negative energy Dirac sea, across zero energy level in presence of gauge
interactions. This was extended to conformal anomaly in \cite{fumita}. In the
present work, we exploit the latter formalism in black hole physics where we
interpret crossing the horizon of black hole (the zero energy level) as a
spectral flow since it is also accompanied by a change of sign in the energy of
the particle. Hence in our formulation the negative energy states below horizon
play a similar role as the Dirac sea. We successfully recover the gravitational
anomaly.
