  The stochastic limit approximation method for ``rapid'' decay is presented,
where the damping rate \gamma is comparable to the system frequency \Omega,
i.e., \gamma \sim \Omega, whereas the usual stochastic limit approximation is
applied only to the weak damping situation \gamma << \Omega. The key formulas
for rapid decay are very similar to those for weak damping, but the dynamics is
quite different. From a microscopic Hamiltonian, the spin-boson model, a Bloch
equation containing two independent time scales is derived. This is a useful
method to extract the minimal dissipative dynamics at high temperature kT >>
\hbar\Omega and the master equations obtained are of the Lindblad form even for
the Caldeira-Leggett model. The validity of the method is confirmed by
comparing the master equation derived through this method with the exact one.
