  We investigate the energy dependence of potentials defined through the
Bethe-Salpeter wave functions. We analytically evaluate such a potential in the
Ising field theory in 2 dimensions and show that its energy dependence is weak
at low energy. We then numerically calculate the nucleon-nucleon potential at
non-zero energy using quenched QCD with anti-periodic boundary condition. In
this case we also observe that the potentials are almost identical at $E\simeq
0$ and $E\simeq 50$ MeV, where $E$ is the center of mass kinetic energy.
