  We extend the study of the light hadron spectrum and the quark mass in
two-flavor QCD to smaller sea quark mass, corresponding to
$m_{PS}/m_{V}=0.60$--0.35. Numerical simulations are carried out using the
RG-improved gauge action and the meanfield-improved clover quark action at
$\beta=1.8$ ($a = 0.2$ fm from $\rho$ meson mass). We observe that the light
hadron spectrum for small sea quark mass does not follow the expectation from
chiral extrapolations with quadratic functions made from the region of
$m_{PS}/m_{V}=0.80$--0.55. Whereas fits with either polynomial or continuum
chiral perturbation theory (ChPT) fails, the Wilson ChPT (WChPT) that includes
$a^2$ effects associated with explicit chiral symmetry breaking successfully
fits the whole data: In particular, WChPT correctly predicts the light quark
mass spectrum from simulations for medium heavy quark mass, such as $m_{PS}/m_V
\simgt 0.5$. Reanalyzing the previous data %at $m_{PS}/m_{V}=0.80$--0.55 with
the use of WChPT, we find the mean up and down quark mass being smaller than
the previous result from quadratic chiral extrapolation by approximately 10%,
$m_{ud}^{\bar{\rm MS}}(\mu=2 {GeV}) = 3.11(17)$ [MeV] in the continuum limit.
