  In the minimal supersymmetric standard model, large left-right mixing of
stau's is sometimes intriguing from phenomenological viewpoints. However, too
large left-right mixing is not acceptable since the electroweak-breaking vacuum
becomes metastable. In this paper the vacuum transition rate is evaluated by
using semiclassical techniques, and constraints on parameters of the model are
shown. In the calculation the bounce solution is derived in the multifield
space. These constraints are also applied to the case of the low-energy minimal
gauge mediation model.
