  The optical-ultraviolet continuum from active galactic nuclei (AGN) seem to
originate from optically thick and/or thin disks, and occasionally from
associated circumnuclear starburst regions. These different possible origins
can, in principle, be discriminated by observations of gravitational
microlensing event. We performed numerical simulations of microlensing of an
AGN disk by a single lensing star passing in front of the AGN. Calculated
spectral variations and light curves show distinct behavior, depending on the
nature of emitting region; the time variation of a few months with strong
wavelength dependence is expected in the case of an optically thick disk ({\it
standard disk}), while an optically thin disk ({\it advection-dominated disk})
will produce shorter, nearly wavelength-independent variation. In the case of a
starburst region much slower variations (over a year) will be superposed on the
shorter variations.
