  We determine the topological susceptibility $\chi_t$ in two-flavor QCD using
the lattice simulations at a fixed topological sector. The topological charge
density is unambiguously defined on the lattice using the overlap-Dirac
operator which possesses exact chiral symmetry. Simulations are performed on a
$16^3 \times 32$ lattice at lattice spacing $\sim$ 0.12 fm at six sea quark
masses $m_q$ ranging in $m_s/6$--$m_s$ with $m_s$ the physical strange quark
mass. The $\chi_t$ is extracted from the constant behavior of the
time-correlation of flavor-singlet pseudo-scalar meson two-point function at
large distances, which arises from the finite size effect due to the fixed
topology. In the small $m_q$ regime, our result of $\chi_t$ is proportional to
$m_q$ as expected from chiral effective theory. Using the formula
$\chi_t=m_q\Sigma/N_f$ by Leutwyler-Smilga, we obtain the chiral condensate in
$N_f=2$ QCD as $\Sigma^{\bar{\mathrm{MS}}}(\mathrm{2 GeV}) = [252(5)(10)
\mathrm{MeV}]^3 $, in good agreement with our previous result obtained in the
$\epsilon$-regime.
