  The possible exotic meson $Z_{c}(3900)$, found in $e^+ e^-$ reactions, is
studied by the method of coupled-channel scattering in lattice QCD. The
interaction among $\pi J/\psi$, $\rho \eta_{c}$ and $\bar{D}D^{*}$ channels is
derived from (2+1)-flavor QCD simulations at $m_{\pi}=410$-$700$ MeV. The
interaction is dominated by the off-diagonal $\pi J/\psi$-$\bar{D}D^{*}$ and
$\rho \eta_{c}$-$\bar{D}D^{*}$ couplings, which indicates that the
$Z_{c}(3900)$ is not a usual resonance but a threshold cusp.
Semi-phenomenological analyses with the coupled-channel interaction are
presented to confirm this conclusion.
