  We report on unprecedented short-term variations detected in the optical flux
from the black hole binary system, V4641 Sgr. Amplitudes of the optical
fluctuations were larger at longer time scales, and surprisingly reached ~60%
around a period of ~10 min. The power spectra of fluctuations are characterized
by a power law. It is the first case in black hole binaries that the optical
emission was revealed to show short-term and large-amplitude variations given
by such a power spectrum. The optical emission from black hole binaries is
generally dominated by the emission from the outer portion of an accretion
disc. The rapid optical fluctuations however indicate that the emission from an
inner accretion region significantly contributes to the optical flux. In this
case, cyclo-synchrotron emission associated with various scales of magnetic
flares is the most promising mechanism for the violently variable optical
emission.
