  The s-wave scattering of $\Sigma_c \bar{D}$ and $\Sigma_c \bar{D}^*$ in the
$I(J^P) = \frac{1}{2}(\frac{1}{2}^-)$ channel is calculated in lattice QCD
using two ensembles with different volumes but the same lattice spacing $a\sim
0.08\mathrm{fm}$ and pion mass $M_\pi \sim 294\mathrm{MeV}$. The scattering
amplitudes near threshold are obtained by L\"uscher's finite volume method. We
find bound state poles in both $\Sigma_c \bar{D}$ and $\Sigma_c \bar{D}^*$
channels, which are possibly related to the $P_c(4312)$ and $P_c(4440) /
P_c(4457)$ pentaquarks observed in experiments. The binding energy is
$6(2)(2)$MeV for $\Sigma_c \bar{D}$ and $7(3)(1)$MeV for $\Sigma_c \bar{D}^*$,
where the first error is the statistical error and the second is the systematic
error due to the lattice artifacts.
