This study investigates future changes in summertime precipitation amounts over the Japanese islands and their relations to the topographical heights by analyzing data from 20 km resolution regional climate model downscalings of MIROC3.2(hires) 20C3M and Special Report on Emission Scenarios A1B scenario data for the periods of 1981–2000 and 2081–2100. Results of the analyses indicate that future increases in June-July-August mean daily precipitation amounts are noticeable in the west and south sides (windward sides) of the mountainous regions, especially in western Japan where heavy rainfall is frequently observed in the recent climate. The large precipitation increases are likely to occur not only in high altitude areas but also at low altitudes where many urban areas are located. In such areas, the occurrence frequencies of precipitation amounts greater than 100 mm d−1 would also increase under the future climate scenario (A1B). One of the main causes of these precipitation changes appears to be the intensification of southwesterly moist air flows in the lower troposphere, which is likely to be associated with future increases in the north-south atmospheric pressure gradient, especially at latitudes south of 35°N. The intensified southwesterly moist air flows that impinge on the western and southern slopes of the mountains can generate stronger upslope flows and well-developed clouds, leading to increased precipitation. In contrast, future precipitation changes in the lee sides of the mountainous regions would be comparatively small. These results indicate that future precipitation changes strongly depend on the topography and prevailing wind direction.