As a result of the introduction of strongly acidic hot spring water from the Tamagawa river system for power source development, Lake Tazawa became an acidic lake and the independent ecosystem is no longer functioning. As a countermeasure, Tamagawa neutralization treatment facility began in 1991 but Lake Tazawa's pH hovers around 5.2 and the effects of the neutralization treatment facility are still unknown. In this study, we conducted a short-term chronic toxicity test using aquatic organisms at three locations in Lake Tazawa-Tamagawa river system: Tamagawa neutralization treatment facility, Tamagawa Dam and Lake Tazawa, and compared before and after adjusting the pH to 6.7. As a result, the toxicity unit (TU) in the downstream direction of the Tamagawa Neutralization Plant, Tamagawa Dam, and Lake Tazawa showed a tendency to decrease, and in particular, the decrease in TU value up to Tamagawa Dam and the immediate improvement in toxicity were significant. Regarding the pH adjustment effect, a positive effect was observed in the treated water of the neutralization treatment facility.