  The RST model is augmented by the addition of a scalar field and a boundary
term so that it is well-posed and local. Expressing the RST action in terms of
the ADM formulation, the constraint structure can be analysed completely. It is
shown that from the view point of local field theories, there exists a hidden
dynamical field in the RST model. Thanks to the presence of this hidden
dynamical field, we can reconstruct the closed algebra of the constraints which
guarantee the general invariance of the RST action. The resulting stress
tensors are recovered to be true tensor quantities. At the quantum level, the
cancellation condition for the total central charge is reexamined. Finally,
with the help of the hidden dynamical field, the fact that the semi-classical
static solution of the RST model has two independent parameters (P,M), whereas
for the classical CGHS model there is only one, can be explained.
