  We have formulated the problem of electron transport through interacting
quantum dot system in the framework of self-consistent perturbation theory, and
show that the current conservation condition is guaranteed due to the gauge
invariant properties of the Green's functions and the generalized Ward
identity. By using a generating functional for the statistics of the
nonequilibrium system, we have obtained general formulae for calculating the
current and the current fluctuations in the presence of arbitrary
time-dependent potentials. As demonstration of application, we have studied the
interaction effects on the finite frequency noise for electron resonant
tunneling through an Anderson impurity, and obtained an analytical equation for
the interaction effect on the finite frequency current noise within the Hartree
approximation, which is an extension of the previous results obtained by
Hershfield on zero frequency shot noise.
