We estimated the contributions of source regions in East Asia to PM_2.5 mass concentrations over Japan for the year 2010 using a regional chemical transport model with an emission sensitivity approach. The horizontal distributions and temporal variations of PM_2.5 concentrations were generally well reproduced by the model. The relative contribution of China to the annual mean PM_2.5 concentration was estimated to be 50–60% in western Japan (from Kyushu to Kinki), and 40% in the Kanto area. Central north China (105°E–124°E, 34°N–42°N) made a particularly significant contribution, accounting for 20–40% of the annual mean PM_2.5 concentration for the whole of Japan. The contribution from foreign sources increased in spring, autumn, and winter in western Japan, and in spring in the Kanto area. The domestic contribution was estimated to be 20–50%. The sum of contributions from foreign anthropogenic sources was greater than the domestic pollution in each receptor region except the Kanto region. The results were obtained from sensitivity simulations with reduced anthropogenic emissions from each source region by 20%. The uncertainties in the source-receptor relationship for annual mean PM_2.5 concentrations were estimated to be within a few percent, from additional sensitivity simulations with different perturbations (a 20% increase and a 50% reduction). The model results suggest that regional-scale transport in East Asia has a significant impact on the attainment of the PM_2.5 environmental standard in Japan.