  We propose a dynamical mechanism of the two-way switching between the
metastable state and the stable state, which has been found in experiments of
photoinduced reversible magnetization and photoinduced structural phase
transition. We find that the two-way switching with a non-symmetry breaking
perturbation such as illumination is possible only in systems with appropriate
parameters. We make it clear that the existence of two time scales in the
dynamical process is important for the two-way switching.
