  We developed a low-mass and high-efficiency charged particle detector for an
experimental study of the rare decay $K_L \rightarrow \pi^0 \nu \bar{\nu}$. The
detector is important to suppress the background with charged particles to the
level below the signal branching ratio predicted by the Standard Model
(O(10$^{-11}$)). The detector consists of two layers of 3-mm-thick plastic
scintillators with wavelength shifting fibers embedded and Multi Pixel Photon
Counters for readout. We manufactured the counter and evaluated the performance
such as light yield, timing resolution, and efficiency. With this design, we
achieved the inefficiency per layer against penetrating charged particles to be
less than $1.5 \times 10^{-5}$, which satisfies the requirement of the KOTO
experiment determined from simulation studies.
