Air pollution in Japan has decreased from the elevated levels found during the period of high economic growth. However, in recent years there is increasing concern regarding air pollution caused by fine particulate matter (PM2.5) and its effect on human health. First, we investigated the seasonal and regional characteristics of air pollution caused by PM2.5 using ground measurements in China and Japan. Next, the cause of air pollution was classified as urban air pollution or transboundary air pollution. Air pollution in Fukuoka (in southwestern Japan) is strongly affected by transboundary air pollution. Nearly half of all air pollution in Tokyo (in eastern Japan) may be urban pollution caused by local emissions. PM2.5 high concentration from winter to spring is often caused by transboundary air pollution. On the other hand, PM2.5 high concentration from summer to autumn is greatly influenced by urban pollution. Air pollution caused by PM2.5 high concentration tends to decrease in Japan. This is due to both a reduction in PM2.5 emissions in Japan and a decrease in PM2.5 concentrations in China.