  We calculate the $\Lambda \Lambda \to YN$ transition rate of
${^{\phantom{\Lambda}6}_{\Lambda \Lambda}}$He by the hybrid picture, the $\pi$
and $K$ exchanges plus the direct quark processes. It is found that the
hyperon-induced decay is weaker than the nucleon-induced decay, but the former
may reveal the short-range mechanism of the weak transition and also give a
clear signal of the strong $\Delta I=3/2$ transition. The $\Lambda \Lambda \to
Y N$ transition in double-$\Lambda$ hypernucleus is complement to the $\Lambda
N \to NN$ transition as it occurs only in the J=0 channel, while the J=1
transition is dominant in the $\Lambda N \to NN$ case.
