  Aperiodic optical variability is a common property of Active Galactic Nuclei
(AGNs), though its physical origin is still open to question. To study the
origin of the optical -- ultraviolet variability in AGN, we compare light
curves of two models to observations of quasar 0957+561 in terms of a structure
function analysis. In the starburst (SB) model, random superposition of
supernovae in the nuclear starburst region produce aperiodic luminosity
variations, while in the disk-instability (DI) model, variability is caused by
instabilities in the accretion disk around a supermassive black hole. We
calculate fluctuating light curves and structure functions, $V(\tau)$, by
simple Monte-Carlo simulations on the basis of the two models. Each resultant
$V(\tau)$ possesses a power-law portion, $[V(\tau)]^{1/2} \propto
\tau^{\beta}$, at short time lags ($\tau$). The two models can be distinguished
by the logarithmic slope, $\beta$; $\beta \sim$ 0.74--0.90 in the SB model and
$\beta \sim$ 0.41--0.49 in the DI model, while the observed light curves
exhibit $\beta \sim$ 0.35. Therefore, we conclude that the DI model is favored
over the SB model to explain the slopes of the observational structure
function, in the case of 0957+561, though this object is a radio-loud object
and thus not really a fair test for the SB model. In addition, we examine the
time-asymmetry of the light curves by calculating $V(\tau)$ separately for
brightening and decaying phases. The two models exhibit opposite trends of
time-asymmetry to some extent, although the present observation is not long
enough to test this prediction.
