  We perform a lattice QCD study of the $\rho$ meson decay from the $N_f=2+1$
full QCD configurations generated with a renormalization group improved gauge
action and a non-perturbatively $O(a)$-improved Wilson fermion action. The
resonance parameters, the effective $\rho\to\pi\pi$ coupling constant and the
resonance mass, are estimated from the $P$-wave scattering phase shift for the
isospin I=1 two-pion system. The finite size formulas are employed to calculate
the phase shift from the energy on the lattice. Our calculations are carried
out at two quark masses, $m_\pi=410\,{\rm MeV}$ ($m_\pi/m_\rho=0.46$) and
$m_\pi=300\,{\rm MeV}$ ($m_\pi/m_\rho=0.35$), on a $32^3\times 64$
($La=2.9\,{\rm fm}$) lattice at the lattice spacing $a=0.091\,{\rm fm}$. We
compare our results at these two quark masses with those given in the previous
works using $N_f=2$ full QCD configurations and the experiment.
