  We analyze the bilayer quantum Hall (QH) system by mapping it to the
monolayer QH system with spin degrees of freedom. By this mapping the tunneling
interaction term is identified with the Zeeman term. We clarify the mechanism
of a spontaneous development of quantum coherence based on the Chern-Simons
gauge theory with the lowest-Landau-Level projection taken into account. The
symmetry group is found to be W_infty*SU(2), which says that the spin rotation
affects the total electron density nearby. Using it extensively we construct
the Landau-Ginzburg theory of the coherent mode. Skyrmion excitations are
topological solitons in this coherent mode. We point out that they are
detectable by measuring the Hall current distribution.
