  We determine the topological susceptibility \chi_t in the
topologically-trivial sector generated by lattice simulations of N_f = 2+1 QCD
with overlap Dirac fermion, on a 16^3 x 48 lattice with lattice spacing ~ 0.11
fm, for five sea quark masses m_q ranging from m_s/6 to m_s (where m_s is the
physical strange quark mass). The \chi_t is extracted from the plateau (at
large time separation) of the 2-point and 4-point time-correlation functions of
the flavor-singlet pseudoscalar meson \eta', which arises from the finite size
effect due to fixed topology. In the small m_q regime, our result of \chi_t
agrees with the chiral effective theory. Using the formula \chi_t =
\Sigma(m_u^{-1} + m_d^{-1} + m_s^{-1})^{-1} by Leutwyler-Smilga, we obtain the
chiral condensate \Sigma^{MSbar}(2 GeV) = [249(4)(2) MeV]^3.
