  Rearrangement effects in light hypernuclei are investigated in the framework
of the Brueckner theory. We can estimate without detailed numerical
calculations that the energy of the $\alpha$-core is reduced by more than 2.5
MeV when the $\Lambda$ adheres to $^4$He to form $^5_\Lambda$He. Similar
assessment of rearrangement contributions is essential to deduce the strength
of $\Lambda\Lambda$ interaction from experimentally observed $\Delta
B_{\Lambda\Lambda}$. The recently observed experimental value of $\sim$ 1 MeV
for the $\Delta B_{\Lambda\Lambda}$ of \hll suggests that the matrix element of
$<\Lambda \Lambda |v|\Lambda \Lambda>$ in \hll is around -2 MeV.
