Electric charge quantization is a long-standing question in particle physics. While fractionally charged particles (millicharged particles hereafter) have typically been thought to preclude the possibility of Grand Unified Theories (GUTs), well-motivated dark-sector models have been proposed to predict the existence of millicharged particles while preserving the possibility for unification. Such models can contain a rich internal structure, providing candidate particles for dark matter. A number of experiments have searched for millicharged particles (𝜒s), but in the parameter space of the charge (𝑄) and mass (𝑚_𝜒), the region of 𝑚_𝜒 > 0.1 GeV/𝑐^2 and 𝑄 < 10^{−3}𝑒 is largely unexplored. SUB-Millicharge ExperimenT (SUBMET) has been proposed to search for sub-millicharged particles 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 expected to be a random coincidence between the two layers due to dark counts in PMTs, which can be reduced significantly using the timing of the proton beam. With 𝑁_{POT} = 5×10^{21}, the experiment provides sensitivity to 𝜒s with the charge
down to 7×10^{−5}𝑒 in 𝑚_𝜒 < 0.2 GeV/𝑐^{2} and 10^{−3}𝑒 in 𝑚_𝜒 < 1.6 GeV/𝑐^2. This is the regime largely uncovered by the previous experiments