  In this paper, standard accretion disk models of AGNs are tested using light
curves of 26 objects well observed for reverberation mapping. Time scales of
variations are estimated by the most common definition of the variability time
scale and the zero-crossing time of the autocorrelation function of the optical
light curves for each source. The measured time scales of variations by the two
methods are consistent with each other. If the typical value of the viscosity
parameter $\alpha \sim 0.1$ is adopted, the measured optical variability time
scales are most close to the thermal time scales of the standard disks. If
$\alpha$ is allowed to range from $\sim 0.03$ to $\sim 0.2$, the measured time
scales are consistent with the thermal time scales of the standard disks. There
is a linear relation between the measured variability time scales and black
hole masses, and this linear relation is qualitatively consistent with
expectation of the standard accretion disk models. The time lags measured by
the ZDCF between different bands are on the order of days. The measured time
lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags
estimated in the case of continuum thermal reprocessing for the standard
accretion disk models. However, the measured time lags of NGC 5548 and Fairall
9 are unlikely to be the case of continuum thermal reprocessing. Our results
are unlikely to be inconsistent with or are likely to be conditionally in favor
of the standard accretion disk models of AGNs.
