  The diffusivity of tagged particles is demonstrated to be heterogeneous on
time scales comparable to or less than the structural relaxation time %taking
place at the interparticle distance in a highly supercooled liquid via 3D
molecular dynamics simulation. The particle motions in the relatively active
regions dominantly contribute to the mean square displacement, giving rise to a
diffusion constant systematically larger than the Stokes-Einstein value. The
van Hove self-correlation function $G_s(r,t)$ is shown to have a large $r$ tail
which can be scaled in terms of $r/t^{1/2}$ for $t \ls 3 \tau_\alpha$, where
$\tau_\alpha \cong$ the stress relaxation time. Its presence indicates
heterogeneous diffusion in the active regions. However, the diffusion process
eventually becomes homogeneous on time scales longer than the life time of the
heterogeneity structure ($\sim 3 \tau_{\alpha}$).
