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 \sim 60 \% around a period of \sim 10 min .
The power spectra of fluctuations are characterized by a power law ( \propto f ^ { - \alpha } , \alpha \sim - 1.7 ) .
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 .