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.
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