  We study a black hole radiation inside the apparent horizon in quantum
gravity. First we perform a canonical quantization for spherically symmetric
geometry where one of the spatial coordinates is dealt as the time variable
since we would like to consider the interior region of a black hole. Next this
rather general formalism is applied for a specific model where the ingoing
Vaidya metric is used as a simple model of an evaporating black hole. Following
Tomimatsu's idea, we will solve analytically the Wheeler-DeWitt equation in the
vicinity of the apparent horizon and see that mass-loss rate of a black hole by
thermal radiation is equal to the result obtained by Hawking in his
semiclassical treatment. The present formalism may have a wide application in
quantum gravity inside the horizon of a black hole such as mass inflation and
strong cosmic censorship etc.
