  The responses of perturbations added into the optically thick,
advection-dominated accretion disk (ADAD), what we call the slim disk (SD), are
investigated through numerical simulations. Although it is proposed that the SD
is thermally stable, I find that a perturbation added into the disk is not
rapidly damped and moves through the disk in its free-fall time. After the
perturbation moves, the global structure of the disk does not vary very much.
These facts may account for the substantial variability of the X-ray
luminosities of stellar super-luminal jet sources (SLJSs) and Narrow-Line
Seyfert 1s (NLS1s).
