During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiroshima and Keihanshin areas were subjected to unusual total rainfall amounts in 72 hours from 1200 UTC 4 July onward. However, the number of sediment disasters was significantly larger in the Hiroshima area. Among the possible reasons for the difference in the sediment disaster occurrences between the Hiroshima and Keihanshin areas, here, we focus on the differences in the rainfall characteristics in these two areas during the heavy rainfall event. Based on the radar observations, we investigate the characteristics of precipitation systems striking the Hiroshima and Keihanshin areas and find that significantly large precipitation systems with areas equal to or larger than 104 km2 were dominant in the Hiroshima area, which caused rapid accumulation of the rainfall amount and enhanced the risk of deadly sediment disasters in this area. On the other hand, in the Keihanshin area, rainfall of moderate intensity and relatively small precipitation systems were found to be dominant. We suggest that the difference in the amount of damage between the Hiroshima and Keihanshin areas was mainly caused by the size difference of the precipitation systems striking these two areas. Statistics relating to the background atmospheric conditions for the precipitation systems in the heavy rainfall event reveal that a high vertical wind shear environment provides preferable conditions for the formation of large precipitation systems.