  In typical examples of the AdS/CFT correspondence, the world-sheet theory
with holes in the presence of D-branes is assumed to be equivalent in a
low-energy limit to a world-sheet theory without holes for a different
background such as $AdS_5 \times S^5$. In the case of the bosonic string, we
claim under the assumption of this equivalence that open string field theory on
$N$ coincident D-branes can be used to provide a nonperturbative definition of
closed string theory based on the fact that the $1/N$ expansion of correlation
functions of gauge-invariant operators reproduces the world-sheet theory with
holes where the moduli space of Riemann surfaces is precisely covered.
