  Following the work of Kawai, Matsuo, and Yokokura, we study the dynamical
collapsing process with spherical symmetry in the time-dependent space-time
background including the back reaction of Hawking radiation. We show that in
this model there are two classes of asymptotic solutions. One of the two
classes is known previously. These states have the slope $\partial a/\partial
r$ approximately equal to 1. The other class of asymptotic solutions is that of
shells with a small thickness. We emphasize that these thin shells should be
properly understood as configurations in the low-energy effective theory. They
behave characteristically differently from the singular states of ideal thin
shells of zero thickness.
