  We report a serendipitous discovery of six very low-luminosity active
galactic nuclei (AGNs) only by optical variability in one-month baseline. The
detected flux variability is ~ 1-5% of the total luminosity of host galaxies.
Careful subtraction of host galaxy components in nuclear regions indicates that
the fractional variability (Delta F / F) of the nuclei is of order unity. At
least one of them is showing a compelling flaring activity within just a few
days, which appears to be quite different from previously known AGN
variability. We obtained spectroscopic data for the one showing the largest
flare and confirmed that it is in fact an AGN at z = 0.33 with an estimated
black hole mass of ~10^8 M_sun. As a possible interpretation, we suggest that
these activities are coming from the region around the black hole event
horizon, which is physically similar to the recently discovered near-infrared
flares of our Galactic nucleus. It is indicated that our Galaxy is not special,
and that surprisingly rapid flaring activity in optical/near-infrared bands may
be commonly hidden in nuclei of apparently normal galaxies with low Eddington
ratios, in contrast to the variability of well-studied luminous AGNs or
quasars.
