Calcite contained in the calcareous clastic rocks of the pre-Miyakoan (i.e., approximately pre-Aptian) formations in the South Kitakami belt has oxygen isotope ratios lower than 18‰ relative to SMOW. Most data converge on a value between 13-16‰, regardless of the age of the formations; &18O is lower than that of recent marine limestone by approximately 15‰. In contrast, calcite from the Miyako Group and post-Miyakoan formations has different isotopic signatures than that from pre-Miyakoan formations. The δ18O values are higher than 17‰ and a tendency towards higher δ18O with decreasing age is notable. The depletion in &18O from calcite in the rocks of both pre- and post-Miyakoan formations is due to isotopic exchange with &18O-poor meteoric water. However, the isotopic alteration was more extensive in the rocks of pre-Miyakoan formations than in the Miyako and post-Miyakoan formations. The Oshima Orogeny at the Barremian Age was accompanied by numerous granite intrusions. It is proposed that the large-scale circulation of meteoric water within the sedimentary formations was set at that period due to the emplacement of hot magmas and that this is the cause of the isotopic alteration found in the South Kitakami Belt.