  Emphasizing that the specification of the representation space or the
quasiparticle picture is essential in nonequilibrium quantum field system, we
have constructed the unique unperturbed representation of the interaction
picture in the superoperator formalism. To achieve it, we put the three basic
requirements (the existence of the quasiparticle picture at each instant of
time, the macroscopic causality and the relaxation to equilibrium). From the
resultant representation follows the formulation of nonequilibrium Thermo Field
Dynamics (TFD). The two parameters, the number distribution and excitation
energy, characterizing the representation, are to be determined by the
renormalization condition. While we point out that the diagonalization
condition by Chu and Umezawa is inconsistent with the equilibrium theory, we
propose a new renormalization condition as a generalization of the on-shell
renormalization on the self-energy which derives the quantum transport equation
and determines the renormalized excitation energy.
