We observed high sulfate ion concentrations in Fukuoka and Osaka city using an aerosol mass spectrometer and an ion chromatograph method in the end of July, 2012. The mass concentration of sulfate ion reached <i>ca.</i> 35 μg m<sup>-3</sup> on 25 July in Fukuoka. The mass concentration of PM<sub>2.5</sub> exceeded 50 μg m<sup>-3</sup> and that of sulfate ion was between 10and 20 μg m<sup>-3</sup> for several hours on 29 July in Osaka. The simulation using Spectral Radiation- Transport Model for Aerosol Species (SPRINTARS) showed that the air mass with a high sulfate ion concentration first covered the Korean peninsula in July 2012 and then spread over the northern part of Kyushu and the Honshu regions of Japan. This indicates that the observed high sulfate ion events were possibly due to the trans-boundary air pollution. Since air pollution with a high sulfate ion concentration potentially causes adverse health effect, the monitoring of trans-boundary air pollution is necessary even in summer.