  A charged-particle veto detector was constructed to study the rare decay
$K_{L} \rightarrow \pi^{0} \nu \bar{\nu}$ in the J-PARC K$^{\mathrm{O}}$TO
experiment. This detector consists of 3mm-thick plastic scintillator strips and
wavelength shifting fibers coupled with MPPCs at the both ends, which makes it
possible to obtain large light output over the wide region. After the
construction and installation to the K$^{\mathrm{O}}$TO experimental area, its
performance was studied using charged particles from $K_{\mathrm{L}}$ decays.
As a preliminary result, we found that the detector had the light yield larger
than 10p.e./100keV and the timing resolution better than 3ns for most regions,
which satisfied the requirements to achieve the standard model
sensitivety($\sim10^{-11}$) for the $K_{L} \rightarrow \pi^{0} \nu \bar{\nu}$
detection.
