  We study the BPS spectrum of four dimensional N=2 SU(2) theory with massive
fundamental matters using the D3-brane probe. Since the BPS states are realized
by string webs subject to the BPS conditions, we determine explicitly the
configurations of such webs. It is observed that there appear BPS string webs
with multiple of junctions corresponding to the fact that the curves of
marginal stability in massive theory are infinitely nested. In terms of the
string configurations, various properties of the curves of marginal stability
are explained intuitively.
