We extend the study of the light hadron spectrum and the quark mass in two-flavor QCD to smaller sea quark mass, corresponding to [Formula Presented]. Numerical simulations are carried out using the RG-improved gauge action and the meanfield-improved clover quark action at [Formula Presented] ([Formula Presented] from [Formula Presented] 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 [Formula Presented]. Whereas fits with either polynomial or continuum chiral perturbation theory (ChPT) fail, the Wilson ChPT (WChPT) that includes [Formula Presented] 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 [Formula Presented]. Reanalyzing the previous data 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%, [Formula Presented] in the continuum limit. © 2004 The American Physical Society.