  We discuss a new mechanism which can solve the crisis in standard big-bang
nucleosynthesis. A long-lived particle $X$ ($10^4 sec \lesssim \tau_X \lesssim
10^6 sec$) which decays into photon(s) will induce cascade photons, and destroy
significant amounts of D and $^3$He without destroying $^4$He or too much
$^7$Li. We numerically investigate this process and derive a constraint on the
properties of $X$ such that the theoretical values of the primordial
light-element abundances agree with observation. We also present some
candidates for the unstable particle $X$.
