Due to economic growth in China, emissions of gaseous components from factories and automobiles have been increasing, which has resulted in severe air pollution. During the winter and spring seasons, Japan, which is on the leeward side of the Asian continent, is on the receiving end of this increasingly problematic transboundary air pollution. In this study, the mass concentration and chemical components of the particulate PM2.5 were continuously observed using an automatic analyzer at Kumamoto on the west coast of Japan from October 2014 to March 2015. A greater number of high PM2.5 days were observed in winter than in autumn. This seasonal change in concentrations was believed to be due to transboundary air pollution traveling from the Asian continent due to seasonal monsoons. The analysis of the chemical composition of PM2.5 supported this idea. The factors leading to high PM2.5 concentrations were investigated and categorized into transboundary air pollution, local air pollution, and volcanic activity based on the analysis of sulfate (SO42–) and sulfur dioxide (SO2) concentrations and model simulations. The average concentration of chemical components showed that local air pollution also influenced air quality in Kumamoto .