  We investigate the D-brane bound states from the viewpoint of the unstable
D/\bar{D}-system and their tachyon condensation. We consider two systems; a
system of k D(-1)-branes and N D3-branes with open strings connecting them and
a system of N D3-branes with open strings corresponding to the k-instanton
flux, both of which are realized through the tachyon condensation from (N+2k)
D3-branes and 2k \bar{D3}-branes with appropriate tachyon profiles. It can be
shown that these systems are related with each other through a unitary gauge
transformation of the D3/\bar{D3}-system. We construct an explicit form of the
gauge transformation and show that the essential elements of the ADHM
construction naturally arise from the explicit form of the gauge
transformation. As a result, the ADHM construction is understood as an outcome
of this gauge equivalence in different low energy limits. The small instanton
singularities can be also understood in this context. Other kinds of solitons
with different codimensions are also discussed from the view point of the
tachyon condensation.
