Among many Cretaceous granitic masses in the Ryoke belt of central Japan, two discordant plutons, Kisokoma and Shinshiro, and the largest batholith, Inagawa, were investigated for the D/H ratio in coexisting biotite and hornblende. The relationships between δD- and X Fe- [= Fe/(Fe + Mg)] values for coexisting biotite and hornblende in the three granitic masses are markedly different. In the Kisokoma pluton, tielines between coexisting biotite and hornblende on the δ D— X Fe diagram are distributed in a narrow area and slopes of the tielines are essentially parallel to that derived from T. Suzuoki and S. Epstein's experimental work. In the Shinshiro pluton the tielines are distributed in two areas though the slopes are also almost parallel to that of Suzuoki and Epstein. In the Inagawa batholith, most of the tielines are nearly vertical with highly scattered δ D-values. The X Fe-values of the minerals are ∼ 0.5 in the Kisokoma and Shinshiro plutons, and ∼ 0.7 in the Inagawa batholith. Such a difference in the δ D— X Fe relationship is explained by the different geological conditions of intrusion of the three granitic masses. Namely, the Kisokoma and Shinshiro plutons are discordant, and the erosion level of the former is shallow and that of the latter is deep. The Inagawa batholith intruded during the geologic movement and interacted with the country rocks. On the basis of Suzuoki and Epstein's experimental work, the δ D-value of the water in a single granitic magma can be estimated to be as low as −60 to −70‰ in the initial stage, and as high as ∼ −30‰ in the later stage, because of the early crystallization of biotite and hornblende, whose δ D-values are usually lower than the value of water in the magma. When the experimental results for hornblende by C.M. Graham et al. and those for biotite by Suzuoki and Epstein are combined, the temperature of the hydrogen exchange equilibration between biotite and magmatic water is calculated to be from 400° to 630°C for the Kisokoma pluton, and from 650° to 800°C for the Shinshiro pluton. It is impossible to estimate the equilibration temperature for the Inagawa batholith, because there are no experimental data on hydrogen isotope fractionation between hydrous silicates, such as biotite and hornblende, with higher X Fe-values than 0.7 and coexisting water.