  Utilizing a non-equilibrium Green function like the generalized Kadanoff-Baym
ansatz, a systematic perturbative method is presented to calculate the
expectation value of an arbitrary physical quantity under the restriction that
the Wigner distribution function is fixed. It is shown that, in the
diagrammatic expression of the quantity, a certain part of contributions can be
eliminated due to the restriction. Together with the quantum kinetic equation,
this method provides a basis for the kinetic-theoretical description.
