  We investigate quasilocal tachyon condensation by using gravity/gauge
duality. In order to cure the IR divergence due to a tachyon, we introduce two
regularization schemes: AdS space and a d=10 Schwarzschild black hole in a
cavity. These provide stable canonical ensembles and thus are good candidates
for the endpoint of tachyon condensation. Introducing the Cardy-Verlinde
formula, we establish the on-shell gravity/gauge duality. We propose that the
stringy geometry resulting from the off-shell tachyon dynamics matches onto the
off-shell AdS black hole, where "off-shell" means non-equilibrium
configuration. The instability induced by condensation of a tachyon behaves
like an off-shell black hole and evolves toward a large stable black hole. The
off-shell free energy and its derivative ($\beta$-function) are used to show
the off-shell gravity/gauge duality for the process of tachyon condensation.
Further, d=10 Schwarzschild black hole in a cavity is considered for the
Hagedorn transition as a possible explanation of the tachyon condensation.
