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 β=6/g2 with four sea quark masses at each β. The lattice spacings at these β's are a≈0.22, 0.16 and 0.11 fm at the physical up and down quark mass, which are fixed by the physical ρ meson mass. The study is supplemented by simulations of pure SU(3) gauge theory with the same gauge action at 5 values of β with lattice spacings 0.09 fm?a ?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 χt1/4= 197-16+13 MeV where the error shows statistical and systematic ones added in quadrature. In full QCD χt at heavy sea quark masses is consistent with that of pure SU(3) gauge theory. A decrease of χt 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 χt from heavy to light sea quark masses is discussed.