Oxygen isotopic ratio in coral skeleton (δ18Oc) is an important parameter to reconstruct past climate in tropical region for several centuries. That is because δ18Oc physically reflects δ18O in sea water (δ18OSW) and δ18OSW is influenced by hydrologic balance such as precipitation, evaporation, and terrestrial runoff at the location. However, the quantitative relationship between δ18OSW and the hydrological balance has not been fully uncovered yet. We predicted δ18OSW with an isotope incorporated atmospheric general circulation model (A-GCM) and compared them with 32 coral records. The model successfully reproduced the δ18OSW recorded in the corals at many sites. We then analyzed what affected the δ18OSW variations. The comparison suggests that coral records represent past hydrologic balance information where interannual variability in precipitation is large. According to the model, the interannual precipitation variation is a dominant factor of the δ18OSW variation, but those of δ18O in both precipitation and in evaporation also have non-negligible influence.