  We investigate the finite size effect on pseudoscalar meson masses and decay
constants using a subset of the "PACS10" configurations which are generated
keeping the space-time volumes over (10 fm$)^4$ in 2+1 flavor QCD at the
physical point. We have tried two kinds of analyses, fixing $\kappa$ values or
measured axial Ward identity quark masses. Comparing the results on (5.4
fm$)^4$ and (10.8 fm$)^4$ lattices, we have found a sizable finite size effect
on the pseudoscalar meson sector in the former analysis: a 2.1(8)%, 4.8(1.6)%,
and 0.36(31)% finite size effect on $m_\pi$, $m_{\rm ud}$, and $f_\pi$,
respectively, on the (5.4 fm$)^4$ lattice. For the latter analysis, the finite
size effect on the pseudoscalar meson decay constants is 0.66(33)% for $f_\pi$,
0.26(13)% for $f_K$, and 0.40(32)% for $f_K/f_\pi$. These values with two-sigma
error bars are consistent with the predictions from the full one-loop SU(3)
chiral perturbation theory, which are 0.20% for $f_\pi$, 0.08% for $f_K$, and
0.13% for $f_K/f_\pi$. The finite size effect on the pseudoscalar meson masses
is hardly detected under the current statistical precision.
