A charged-particle veto detector was constructed to study the rare decay $K_{L} rightarrow pi^{0} u bar{ u}$ in the J-PARC K$^{mathrm{O}}$TO experiment. This detector consists of 3mm-thick plastic scintillator strips and wavelength shifting fibers c
oupled 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 $K_{L} rightarrow pi^{0} u bar{ u}$ detection.
