Air pollution is problem of deep concern to human health. In Japan, the air pollution levels experienced during the recent period of rapid economic growth have been reduced. However, fine particulate matter (PM2.5) has not yet reached the environmental standards at many monitoring stations. The Japanese environmental quality standard for PM2.5 that was ratified in 2009 lags about four decades behind other air pollutants, including sulfur dioxide, nitrogen dioxide, carbon monoxide, photochemical oxidants, and suspended particulate matter. Recently, trans-national air pollutants have been observed to cause high concentrations of PM2.5 in Japan. To obtain wide distribution of PM2.5, the satellite based PM2.5 products are extremely useful. We investigate PM2.5 concentrations measured using ground samplers in Japan and the satellite based PM2.5 products, taking into consideration various geographical and weather conditions.