We have used the terrestrial cosmogenic nuclides and geochemical mass balance method to estimate the long-term chemical weathering rate at nine small catchments of the Ashima River basin in the Northern Japanese Alps, central Japan. The basin consists of steep slopes (25-40°) and has a high relief of about 1500 m (800-2300 m a.s.l.). Climate varies from humid temperate at the 800-m bottom of the basin with a mean annual temperature of 9.5℃ and an annual rainfall of 1000-1500 mm to a cold environment with a mean annual temperature of 0.2℃ at the 2300-m divide. In situ ^<26>Al of stream sediment in these catchments showed that denudation rate ranges from 200 to 2000 mm ky^<-1>, which varies with the mean slope of a catchment. We calculated an index of the chemical depletion fraction (CDF), which is defined as the ratio of the chemical weathering rate to denudation rate and using a concentration of an immobile element, titanium, in bedrock and stream sediment with 0.25-2.0 mm grains. Analysis of stream sediment for each catchment showed that CDF ranges from 0.08 to 0.6 and chemical weathering rates range from 100 to 400 mm ky^<-1>. The fraction decreases with an increasing denudation rate and altitude, which implies that both climate and erosion are contributing factors. In the two upstream catchments at 1800 m a.s.l., the catchment with a higher denudation rate of 2000 mm ky^<-1> has a lower CDF (0.08), which indicates the effect of erosion on this weathering. While the rate increases with increasing denudation rate of 100-1000 mm ky^<-1>, it tends to decline at an extremely high denudation rate (> 1000 mm ky^<-1>). This turndown in chemical weathering rate may reflect a transition from supply-limited to kinetically-limited chemical weathering of hillslope materials in mountainous basins.