  A systematic method to obtain the effective Lagrangian on the BPS background
in supersymmetric gauge theories is worked out, taking domain walls and
vortices as concrete examples. The Lagrangian in terms of the superfields for
four preserved SUSY is expanded in powers of the slow-movement parameter
lambda. The expansion gives the superfield form of the BPS equations at
{O}(lambda^0), and all the fluctuation fields at {O}(lambda^1). The density of
the Kaehler potential for the effective Lagrangian follows as an automatic
consequence of the lambda expansion with manifest (four preserved) SUSY.
