We apply chiral perturbation theory to the pseudoscalar meson mass and decay<br />
constant data obtained in the PACS-CS Project toward 2+1 flavor lattice QCD<br />
simulations with the O(a)-improved Wilson quarks. We examine the existence of<br />
chiral logarithms in the quark mass range from m_{ud}=47 MeV down to 6 MeV on a<br />
(2.8 fm)^3 box with the lattice spacing a=0.09 fm. Several low energy constants<br />
are determined. We also discuss the magnitude of finite size effects based on<br />
chiral perturbation theory.