  The momentum distribution of relative motion between two nucleons gives
information on the correlation in nuclei. The momentum distribution is
calculated for both $^{6}$He and $^6$Li which are described in a three-body
model of $\alpha$+$N$+$N$. The ground state solution for the three-body
Hamiltonian is obtained accurately using correlated basis functions. The
momentum distribution depends on the potential model for the $N$-$N$
interaction. With use of a realistic potential, the $^6$He momentum
distribution exhibits a dip around 2 fm$^{-1}$ characteristic of $S$-wave
motion. In contrast to this, the $^6$Li momentum distribution is very similar
to that of the deuteron; no dip appears because it is filled with the $D$-wave
component arising from the tensor force.
