  We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to
\pi^0 \nu \overline{\nu}$, in data from the first 100 hours of physics running
in 2013 of the J-PARC KOTO experiment. One candidate event was observed while
$0.34\pm0.16$ background events were expected. We set an upper limit of
$5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level
(C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L}
\to \pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an
invisible particle with a mass of 135 MeV/$c^{2}$.
