We propose a 100 ksec Suzaku observation of the Galactic microquasar V4641 Sgr in an outburst. V4641 Sgr has many unique charactersitics:1)giant X-ray outbursts with fast rise and decay times, 2)violent variability in the X-ray and optical bands, and 3)remarkable iron-K disk-line profiles in the X-ray spectrum. However, it is hardly understood due to the poor X-ray coverage which is partly the result of the short outburst duration and rapid intensity variations of this source. Hence, we collaborate closely with VSNET and RXTE/ASM team for the rapid trigger. The moderate energy resolution and wideband energy coverage of Suzaku enable us to clarify the radiation mechanisms of V4641 Sgr. This observation is now planed simultaneously with RXTE and many optical-near IR observatories.