  We explore flavor singlet pseudoscalar meson, eta', spectrum in two-flavor
(Nf=2) lattice QCD. The continuum-like relation between the topology of the QCD
vacuum and the U(1)_A anomaly, that prevents eta' meson from being a would-be
Nambu-Goldstone boson, are expected to hold in the domain wall fermions (DWF)
used as a lattice quark field in this work. Although our simulation is limited
to relatively heavy quark masses and statistical error is not magnificently
small despite of improvements in measurements and fit procedures for meson
propagators, we obtained m_eta'=819(127) MeV for Nf=2 QCD, where the error is
statistical only. Potentially serious systematic error in the number are
discussed. Results for other meson are also reported.
