  In the framework of the Schwinger-Dyson equation and the Bethe-Salpeter
equation in the improved ladder approximation, we calculate the pi^+ - pi^0
mass difference on the same footing as the pion decay constant and the QCD S
parameter (or L_{10}) through the difference between the vector current
correlator \Pi_{VV} and the axial-vector current correlator \Pi_{AA}. We find
that all the results can be fit to the experimental values for rather large
Lambda_{QCD} \sim 700 MeV which reflects the ``scale ambiguity''. By fitting to
the calculated data using the pole saturated form of \Pi_{VV} - \Pi_{AA}, we
also derive masses and decay constants of rho meson and a_1 meson, which we
found are consistent with the experiments rather insensitively to the ``scale
ambiguity''.
