We report on the detection of an optical outburst which preceded the X-ray main flare of the fast X-ray nova V4641 Sagittarii. The object became active since early August 1999, and then experienced an optical outburst reaching 8.8 mag on 1999 September 15. The observed maximum was between two X-ray flares, that is, 12.1 hr after the X-ray sub-flare and 7.4 hr before the main one. The brightness returned to the quiescent level within 5 d, which indicates that the optical outburst rapidly faded as those at other wavelengths, while typical X-ray novae exponentially decay with an $e$-folding time of 30–40 d. We confirmed ellipsoidal modulations in our observations at quiescence. Using them with the previously reported observations, we calculated the period of modulations to be $2.817284\pm 0.000018 \,\mathrm{d}$. From the $\mathrm{H}\alpha$ absorption line in quiescent spectra, we obtained the semi-amplitude of the radial velocity for the secondary star to be $176 \pm 20 \,\mathrm{km} \,\mathrm{s}^{-1}$. Assuming the mass ratio reported in Orosz et al. (2001, ApJ, 555, 489) we estimated the lower limit of the compact object mass to be $5.84 \,{{{M}_{\odot}}}$. To interpret the short outburst and the large luminosity during the outburst, we propose a scenario of a transient supercritical accretion onto a black hole.