  We present the results of the detailed analysis of an optical imaging survey
conducted using the Subaru / Hyper Suprime-Cam (HSC), which aims to identify an
optical counterpart to the gravitational wave event GW151226. In half a night,
the $i$- and $z$-band imaging survey by HSC covers 63.5deg$^2$ of the error
region, which contains about 7\% of the LIGO localization probability, and the
same field is observed in three different epochs. The detectable magnitude of
the candidates in a differenced image is evaluated as $i \sim 23.2$ mag for the
requirement of at least two 5$\sigma$ detections, and 1744 candidates are
discovered. Assuming a kilonova as an optical counterpart, we compared the
optical properties of the candidates with model predictions. A red and rapidly
declining light curve condition enables the discrimination of a kilonova from
other transients, and a small number of candidates satisfy this condition. The
presence of stellar-like counterparts in the reference frame suggests that the
surviving candidates are likely to be flare stars. The fact that most of those
candidates are in galactic plane, $|b|<5^{\circ}$, supports this
interpretation. We also checked whether the candidates are associated with the
nearby GLADE galaxies, which reduces the number of contaminants even with a
looser color cut. When a better probability map (with localization accuracy of
$\sim50{\rm deg}^2$) is available, kilonova searches of up to approximately
$200$ Mpc will become feasible by conducting immediate follow-up observations
with an interval of 3--6 days.
