  The first detected gravitational wave from a neutron star merger was
GW170817. In this study, we present J-GEM follow-up observations of SSS17a, an
electromagnetic counterpart of GW170817. SSS17a shows a 2.5-mag decline in the
$z$-band from 1.7 days to 7.7 days after the merger. Such a rapid decline is
not comparable with supernovae light curves at any epoch. The color of SSS17a
also evolves rapidly and becomes redder for later epochs; the $z-H$ color
changed by approximately 2.5 mag in the period of 0.7 days to 7.7 days. The
rapid evolution of both the optical brightness and the color are consistent
with the expected properties of a kilonova that is powered by the radioactive
decay of newly synthesized $r$-process nuclei. Kilonova models with Lanthanide
elements can reproduce the aforementioned observed properties well, which
suggests that $r$-process nucleosynthesis beyond the second peak takes place in
SSS17a. However, the absolute magnitude of SSS17a is brighter than the expected
brightness of the kilonova models with the ejecta mass of 0.01 $\Msun$, which
suggests a more intense mass ejection ($\sim 0.03 \Msun$) or possibly an
additional energy source.
