  We study black hole radiation inside black holes within the framework of
quantum gravity. First, we review on our previous work of a canonical
quantization for a spherically symmetric geometry where one of the spatial
coordinates is treated as the time variable, since we think of the interior
region of a black hole. Based on this formalism, under physically plausible
assumptions, we solve the Wheeler-De Witt equation inside the black hole, and
show that the mass-loss rate of an evaporating black hole due to thermal
radiation is equivalent to the result obtained by Hawking in his semi-classical
approach. A remarkable point is that our assumptions make the momentum
constraint coincide with the Hamiltonian constraint up to an irrelevant overall
factor. Furthermore, for comparison, we solve the Wheeler-De Witt equation
outside the black hole as well, and see that the mass-loss rate of an
evaporating black hole has the same expression. The present analysis suggests
that the black hole radiation comes from the black hole singularity. We also
comment on the Birkhoff theorem in quantum gravity.
