In this study, we have determined the H2O content of arc andesitic magmas erupted from three volcanoes in Japan [the 1929 A.D. eruption of Hokkaido-Komagatake (Ko-a), Tenmei eruption of Asama (As-A), and Taisho eruption of Sakurajima volcanoes (Sz-Ts/P1)]. The amount of H2O dissolved in the melt at magma chamber was estimated by analyzing the H2O content of clinopyroxene phenocrysts and using the partition coefficient of H2O between melt and clinopyroxene. On the basis of infrared spectroscopic analyses, the H2O contents of clinopyroxene were 329 ± 65, 247 ± 72, and 213 ± 36 ppm for Ko-a, As-A, and Sz-Ts/P1, respectively. The partition coefficient was calculated from the H2O content of Ko-a clinopyroxene and that of rhyolitic melt inclusions included in clinopyroxene (3.1 wt% H2O). Assuming that clinopyroxene was in equilibrium with melt inclusions, the partition coefficient is 0.011 ± 0.003, which results in H2O contents of the melts at the magma chamber of 2.3 ± 0.9 and 2.0 ± 0.6 wt% for As-A and Sz-Ts/P1, respectively. The H2O content of magma in the magma chamber calculated at saturation on the basis of magma-chamber depth estimated from geophysical observation shows good agreement with the measured value for As-A; those of Ko-a and Sz-Ts/P1 are slightly higher than the measured values. These results indicate that arc andesitic magmas are saturated with H2O or are ready to saturate before the eruptions.