The size distribution and removal efficiency of particulate matter (PM) as primary particles from three gasification and melting furnaces for municipal solid waste (MSW) with state-of-the-art technologies for air pollution control devices (APCDs) were investigated by direct PM sampling using a nine-stage cascade impactor to show emissions and impact of PM2.5 from thermal treatment of MSW in Japan. PM distributions showed that most PM was fine particles, and that coarse particles were more efficiently removed than fine particles. PM and PM2.5 concentrations at the APCD inlets were 1312 ± 63 and 794 ± 119 mg/Nm3, and 1415 ± 225 and 763 ± 327 mg/Nm3 from shaft-type furnaces and fluidized bed and swirl-flow furnaces, respectively. The removal efficiencies of PM and PM2.5 by APCDs were over 99.96% in all plants, and one plant with a bag filter-selective catalytic reduction (BF-SCR) system and another with a BF-wet scrubber-SCR-BF system achieved over 99.99%. PM and PM2.5 emissions from gasification and melting of MSW in Japan were 8.85 and 5.52 Mg/yr, respectively, contributing 3.0% and 2.5% of total PM and PM2.5 emissions from thermal treatment of MSW in Japan. The PM and PM2.5 emissions were effectively controlled by state-of-the-art APCDs.