In order to investigate the acid rain formation under the coexistence of SO₂(g), H₂O₂(g), and HNO₃(g) in the air, a mathematical model has been built and some numerical simulations have been carried out with use of the model. The simulation reveals that SO₂(g) absorbed into a raindrop is released and then re-absorbed as the fall distance increases. The desorption and re-absorption processes of SO₂(g) are caused by: (1) the fact that the equilibrium concentration of H₂O₂(aq) and HNO₃(aq) in raindrops are much higher than that of SO₂(aq), and (2) the fact that the oxidation reaction rate of HSO₃ ^? with H₂O₂(aq) increases with H⁺ concentration in raindrops. The degree of acidification of the rainwater has been estimated by introducing a raindrop size distribution. The acidification is mainly caused by the adsorption of SO₂(g) in the usual case where the atmospheric concentration of SO₂(g) is much higher than that of HNO₃(g). With the increase in the atmospheric concentration of HNO₃(g), the concentration of H⁺ generated from SO₂(g) decreases and the contribution of HNO₃(g) to the generation of H⁺ becomes dominant.