  We investigate the axial U(1) anomaly of two-flavor QCD at temperatures
190--330 MeV. In order to preserve precise chiral symmetry on the lattice, we
employ the Mobius domain-wall fermion action as well as overlap fermion action
implemented with a stochastic reweighting technique. Compared to our previous
studies, we reduce the lattice spacing to 0.07 fm, simulate larger multiple
volumes to estimate finite size effect, and take more than four quark mass
points, including one below physical point to investigate the chiral limit. We
measure the topological susceptibility, axial U(1) susceptibility, and examine
the degeneracy of U(1) partners in meson and baryon correlators. All the data
above the critical temperature indicate that the axial U(1) violation is
consistent with zero within statistical errors. The quark mass dependence
suggests disappearance of the U(1) anomaly at a rate comparable to that of the
SU(2)_L x SU(2)_R symmetry breaking.
