  We studied the feasibility of an experiment searching for sub-millicharged
particles ($\chi$s) using 30 GeV proton fixed-target collisions at J-PARC. The
detector is composed of two layers of stacked scintillator bars and PMTs and is
proposed to be installed 280 m from the target. The main background is a random
coincidence between two layers due to dark counts in PMTs, which can be reduced
to a negligible level using the timing of the proton beam. With
$N_\textrm{POT}=10^{22}$ which corresponds to running the experiment for three
years, the experiment provides sensitivity to $\chi$s with the charge down to
$5\times10^{-5}$ in $m_\chi<0.2$ $\textrm{GeV}/\textrm{c}^2$ and
$8\times10^{-4}$ in $m_\chi<1.6$ $\textrm{GeV}/\textrm{c}^2$. This is the
regime largely uncovered by the previous experiments. We also explored a few
detector designs to achieve an optimal sensitivity to $\chi$s. The
photoelectron yield is the main driver, but the sensitivity does not have a
strong dependence on the detector configuration in the sub-millicharge regime.
