  The Outer-Edge Veto (OEV) counter subsystem for extra-photon detection from
the backgrounds for the? $K^0_L\rightarrow\pi^0\nu\bar{\nu}$ decay is located
at the outer edge of the endcap CsI calorimeter of the KOTO experiment at
J-PARC. The subsystem is composed of 44 counters with different cross-sectional
shapes. All counters are made of lead and scintillator plates and read out
through wavelength-shifting fibers. In this paper, we discuss the design and
performances of the OEV counters under heavy load ($\sim8$ tons/m$^2$) in
vacuum. For 1-MeV energy deposit, the average light yield and time resolution
are 20.9 photo-electrons and 1.5 ns, respectively. Although no pronounced peak
by minimum-ionizing particles is observed in the energy distributions, an
energy calibration method with cosmic rays works well in monitoring the gain
stability with an accuracy of a few percent.
