Although the amount of acidic deposition has recently decreased in Japan, acid deposition has deteriorated some forest ecosystems during the past few decades. We investigated the yearly variations in stream water chemistry for more than 20 years in two areas (Yusuhara and Taisho) in the Shmanto River Basin, southern Japan, where the effects of acidic deposition are considered to be modest. Stream water samples were collected monthly from three forest watersheds selected at each site. The annual means of the stream chemistry were predicted by multiple regression analysis. The sunlight hours were positively related with the potassium, magnesium, calcium, nitrate, sulfate, and bicarbonate concentrations in stream water. The results suggest that long sunlight hours boost the photosynthetic activities, thus promoting soil respiration and decomposition of soil organic matter; moreover, a higher carbonic acid concentration in the soil solution promotes cation weathering and carbonic acid dissociation to bicarbonate. The ammonium, nitrate, and sulfate concentrations in the bulk precipitation have decreased at Yusuhara and the sodium, magnesium, calcium, chloride, nitrate, and sulfate concentrations in the stream water have decreased in both areas. The nitrate and sulfate concentrations apparently responded to the decreasing input of acidic deposition. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previous inputs of acidic deposition enhanced the rate of rock weathering.