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The black hole binary V404 Cygni (= GS 2023+338) had an outburst on June 15,2015 after 26 years of quiescence. We report the multi-color opticalobservation (g', Rc, and Ic) of this object at the beginning of its outburstperformed by the MITSuME 50cm telescope in Akeno, Yamanashi, and theMURIKABUSHI 105 cm Telescope at Ishigakijima Astronomical Observatory. Withtime domain analysis of the multi-color light curves, we successfullydecomposed optical variations into two components: a highly-variable, and alittle-variable component. The loci of the little-variable component in thecolor-color diagram are consistent with that of the multi-temperature blackbodyradiation, while those of the highly-variable component trace out a power-lawspectrum with a spectral index {\alpha}~0.6-1.0. For the little-variablecomponent, we argue that an irradiated disk with the innermost temperaturehigher than ~2.0$\times10^4$ K and the outermost temperature lower than~6.5$\times10^3$ K is the most plausible source. The gradual rise trend of thelight curve during our observation is probably due to the growing of thelittle-variable component. The observed spectral energy distribution (SED) fromthe optical to ultraviolet can be expressed by a model consisting of apower-law component and an irradiated disk component.

MITSuME observation of V404 Cygni in the 2015 outburst: Two optical variable components with different variability