Using chemical data of highly acidic Tamagawa Hot Spring, Akita Prefecture, NE Japan,  upwelling and geochemical processes of geothermal fluid in the central region of volcanic–hydrothermal system were examined through geochemical calculation. The Tamagawa hot springs include Cl-rich acid SO4–Cl liquid discharges with a slight temporal variation in S concentration. The calculation indicated deep solutions at 300 °C having Cl concentrations of 98–100 mmolal and S concentrations of 59–64 mmolal at SO2/H2S log gas ratios of -0.86 to -0.46 under an assumption of fluid ascent after boiling and mixing twice. The temporal variation in the S of the discharges might be attributed to the input of S from the magmatic vapor to the hydrothermal system. The calculation tentatively required a double-fluid flow through the conduit and surrounding wall rock and sequential mixing of the fluids. This study revealed the upflow condition coupled with geochemical evolution of geothermal fluids in the central region of volcanic–hydrothermal systems as the geothermal exploitation.