  Spin-glass (SG) states of the 3-dimensional Ising Edwards-Anderson model
under a static magnetic field $h$ are examined by means of the standard Monte
Carlo simulation on the field-shift aging protocol at temperature $T$. For each
process with $(T; \tw, h)$, $\tw$ being the waiting time before the field is
switched on, we extract the dynamical crossover time, $\tcr(T; \tw, h)$. We
have found a nice scaling relation between the two characteristic length scales
which are properly determined from $\tcr$ and $\tw$ and then are normalized by
the static field crossover length introduced in the SG droplet theory. This
scaling behavior implies the instability of the SG phase in the equilibrium
limit even under an infinitesimal $h$. In comparison with this numerical result
the field effect on real spin glasses is also discussed.
