  We present our recent lattice calculation with dynamical quarks using the
overlap fermion formulation, which has exact chiral symmetry. It is possible to
compare our data of meson mass and decay constant with the prediction from the
chiral perturbation theory.
  From such comparison, we investigate the convergence property of the chiral
expansion. For $N_f=2$, we observe that the prediction to NLO does not converge
at the scale of kaon mass. Based on this fact, we extend the analysis to the
$N_f=2+1$ case and carry out the extrapolation to the physical mass point using
the NNLO formulae.
