  We derive dissipative effective Hamiltonian for the unstable Lee model
without any ad hoc coarse graining procedure. Generalized radiative
corrections, utilizing the in-in formalism of quantum field theory,
automatically yield irreversibility as well as the decay of quantum coherence.
Especially we do not need to extend the ordinary Hilbert space for describing
the intrinsically dissipative system if we use the generalized in-in formalism
of quantum field theory.
