  We aim to construct from first principles a perturbative framework for
studying nonequilibrium quantum-field systems that include massless Dirac
fermions. The system of our concern is quasiuniform system near equilibrium or
nonequilibrium quasistationary system. We employ the closed-time-path formalism
and use the so-called gradient approximation. Essentially no further
approximation is introduced. We construct a fermion propagator, with which a
well-defined perturbative framework is formulated. In the course of
construction of the framework, we obtain the generalized Boltzmann equation
(GBE) that describes the evolution of the number-density functions of
(anti)fermionic quasiparticles.
