  We construct the six-dimensional topological field theory appropriate to
describe the ground-state configurations of D5-branes. A close examination on
the degenerations of D5-branes gives us the physical observables which can be
regarded as the Poincar\'e duals of the cycles of the moduli space. These
observables are identified with the creation opeartors of the bound states of
D5-branes and lead to the second quantization of five-branes. This
identification of the bound states with the cycles also provides their
topological stability and suggests that the bound states of five-branes have
internal structures. The partition function of the second-quantized five-branes
is also discussed.
