Entropy of a nonstatic black hole

We point out that the brick-wall model cannot be applied to the nonstatic black hole. In the case of a static hole, we propose a new model where the black hole entropy is attributed to the dynamical degrees of the field covering the two-dimensional membrane just outside the horizon. A cutoff different from the model of 't Hooft is necessarily introduced. It can be treated as an increase in horizon because of the space-time fluctuations. We also apply our model to the nonequilibrium and nonstatic cases, such as Schwarzschild-de Sitter and Vaidya space-times. In the nonstatic case, the entropy relies on a time-dependent cutoff.