We propose a new experiment sensitive to the detection of millicharged particles produced at the $30$ GeV proton fixed-target collisions at J-PARC. The potential site for the experiment is B2 of the Neutrino Monitor building, $280$ m away from the target. With $textrm{N}_textrm{POT}=10^{22}$, the experiment can provide sensitivity to particles with electric charge $3times10^{-4},e$ for mass less than $0.2$ $textrm{GeV}/textrm{c}^2$ and $1.5times10^{-3},e$ for mass less than $1.6$ $textrm{GeV}/textrm{c}^2$. This brings a substantial extension to the current constraints on the charge and the mass of such particles.
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