  We present results for light quark masses from a systematic lattice QCD study
with two degenerate flavors of dynamical quarks. Simulations are made with a
renormalization-group improved gauge action and a mean-field improved clover
quark action for sea quark masses corresponding to $m_{\rm PS}/m_{\rm V}
\approx 0.8$--0.6 and the lattice spacing $a=0.22$--0.11 fm. In the continuum
limit we find $m_{ud}^{\bar{MS}}(2 {\rm GeV})=3.44^{+0.14}_{-0.22}$ MeV using
the $\pi$ and $\rho$ meson masses as physical input, and $m_s^{\bar{MS}}(2 {\rm
GeV})=88^{+4}_{-6}$ MeV or $90^{+5}_{-11}$ MeV with the $K$ or $\phi$ meson
mass as additional input. The quoted errors represent statistical and
systematic combined, the latter including those from continuum and chiral
extrapolations, and from renormalization factors. Compared to quenched results,
two flavors of dynamical quarks reduce quark masses by about 25%.
