We study black hole radiation inside the apparent horizon in quantum g
ravity. First we perform a canonical quantization for a spherically sy
mmetric geometry, where one of the spatial coordinates is treated as t
he time variable, since we would like to consider the interior region
of a black hole. Next this rather general formalism is applied for a s
pecific model, where the ingoing Vaidya metric is used as a simple mod
el of an evaporating black hole. Following Tomimatsu's idea, we will a
nalytically solve the Wheeler-DeWitt equation in the vicinity of the a
pparent horizon and see that the mass-loss rate of a black hole due to
thermal radiation is equal to the result obtained by Hawking in his s
emiclassical treatment. The present formalism may have a wide applicat
ion in quantum gravity inside the horizon of a black hole to problems
such as mass inflation, strong cosmic censorship, etc.