  The coupled equations which describe the temporal evolution of the
Bose-Einstein condensed system are derived in the framework of nonequilibrium
Thermo Field Dynamics. The key element is that they are not the naive
assemblages of presumable equations, but are the self-consistent ones derived
by appropriate renormalization conditions. While the order parameter is
time-dependent, an explicit quasiparticle picture is constructed by a
time-dependent expansion. Our formulation is valid even for the system with a
unstable condensate, and describes the condensate decay caused by the Landau
instability as well as by the dynamical one.
