  Through introducing a notion of renormalization of particle-number density, a
simple perturbation scheme of nonequilibrium quantum-field theory is proposed.
In terms of the renormalized particle-distribution functions, which
characterize the system, the structure of the scheme (and then also the
structure of amplitudes and reaction rates) are the same as in the equilibrium
thermal field theory. Then, as an obvious consequence, the amplitudes and
reaction rates computed in this scheme are free from pinch singularities due to
multiple products of $\delta$-functions, which inevitably present in
traditional perturbation scheme.
