  We show that the density matrix for a finite conductor attached to reservoirs
obtained by Keldysh formalism is of MacLennan-Zubarev form. On the basis of the
fact that the density matrix is the invariant part proposed by Zubarev, it is
shown that Keldysh formalism may describe the irreversible processes and
steady-state feature of the system. An important consequence of the
MacLennan-Zubarev form of the density matrix is a generalization of the Kubo
formula in a nonequilibrium case. On the basis of the result, we propose the
formula of shot noise and a nonequilibrium identity between differential
conductance, noise power and shot noise as a generalized Nyquist-Johnson
relation.
