  An exploratory study is made of the nucleon-nucleon $s$-wave scattering
lengths in quenched lattice QCD with the Wilson quark action. The $\pi$-$N$ and
$\pi$-$\pi$ scattering lengths are also calculated for comparison. The
calculations are made with heavy quarks corresponding to $m_\pi/m_\rho\approx
0.73-0.95$. The results show that the $N$-$N$ system has an attractive force in
both spin-singlet and triplet channels, with their scattering lengths
significantly larger than those for the $\pi$-$N$ and $\pi$-$\pi$ cases, a
trend which is qualitatively consistent with the experiment. Problems toward a
more realistic calculation for light quarks are discussed.
