  For the attractive interaction, the L\"uscher's finite volume formula gives
the phase shift at negative squared moment $k^2<0$ for the ground state in the
finite volume, which corresponds to the analytic continuation of the phase
shift at $k^2<0$ in the infinite volume. Using this fact, we reexamine
behaviors of phase shifts at $k^2 <0$ obtained directly from plateaux of
effective energy shifts in previous lattice studies for two nucleon systems on
various volumes. We have found that data, based on which existences of the
bound states are claimed, show singular behaviors of the phase shift at
$k^2<0$, which seem incompatible with smooth behaviors predicted by the
effective range expansion. This, together with the fake plateau problem for the
determination of the energy shift, brings a serious doubt on existences of the
$NN$ bound states claimed in previous lattice studies at pion masses heavier
than 300 MeV.
