  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 optical
observation (g', Rc, and Ic) of this object at the beginning of its outburst
performed by the MITSuME 50cm telescope in Akeno, Yamanashi, and the
MURIKABUSHI 105 cm Telescope at Ishigakijima Astronomical Observatory. With
time domain analysis of the multi-color light curves, we successfully
decomposed optical variations into two components: a highly-variable, and a
little-variable component. The loci of the little-variable component in the
color-color diagram are consistent with that of the multi-temperature blackbody
radiation, while those of the highly-variable component trace out a power-law
spectrum with a spectral index {\alpha}~0.6-1.0. For the little-variable
component, we argue that an irradiated disk with the innermost temperature
higher 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 the
light curve during our observation is probably due to the growing of the
little-variable component. The observed spectral energy distribution (SED) from
the optical to ultraviolet can be expressed by a model consisting of a
power-law component and an irradiated disk component.
