  We present a study of the topological susceptibility in lattice QCD with two
degenerate flavors of dynamical quarks. The topological charge is measured on
gauge configurations generated with a renormalization group improved gauge
action and a mean field improved clover quark action at three values of
$\beta=6/g^2$, corresponding to lattice spacings of $a \approx 0.22$, 0.16 and
0.11 fm, with four sea quark masses at each $\beta$. The study is supplemented
by simulations of pure SU(3) gauge theory with the same gauge action at 5
values of $\beta$ with lattice spacings 0.09 fm$\simlt a \simlt$0.27 fm. We
employ a field theoretic definition of the topological charge together with
cooling. For the topological susceptibility in the continuum limit of pure
SU(3) gauge theory we obtain $\chi_t^{1/4} = 197^{+13}_{-16}$ MeV where the
error shows statistical and systematic ones added in quadrature. In full QCD
$\chi_t$ at heavy sea quark masses is consistent with that of pure SU(3) gauge
theory. A decrease of $\chi_t$ toward light quark masses, as predicted by the
anomalous Ward-Takahashi identity for U(1) chiral symmetry, becomes clearer for
smaller lattice spacings. The cross-over in the behavior of $\chi_t$ from heavy
to light sea quark masses is discussed.
