  We present the first search for the pair production of dark particles $X$ via
$K_L^0\to XX$ with $X$ decaying into two photons using the data collected by
the KOTO experiment. No signal was observed in the mass range of 40 - 110
MeV/c$^2$ and 210 - 240 MeV/c$^2$. This sets upper limits on the branching
fractions as $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-4) $\times$ 10$^{-7}$ and
$\mathcal{B}(K_L^0 \to XX)$ $<$ (1-2) $\times$ 10$^{-6}$ at the 90% confidence
level for the two mass regions, respectively.
