  Modification of standard big-bang nucleosynthesis is considered in the
minimal supersymmetric standard model to resolve the excessive theoretical
prediction of the abundance of primordial lithium 7. We focus on the stau as a
next-lightest superparticle, which is long lived due to its small mass
difference with the lightest superparticle. It provides a number of additional
decay processes of $\mathrm{^{7}Li}$ and $\mathrm{^{7}Be}$. A particularly
important process is the internal conversion in the stau-nucleus bound state,
which destroys the $\mathrm{^{7}Li}$ and $\mathrm{^{7}Be}$ effectively. We show
that the modification can lead to a prediction consistent with the observed
abundance of $\mathrm{^{7}Li}$.
