[en] Speleothem "1"4C has recently emerged as a potentially powerful proxy for hydrology changes in comparison with atmospheric "1"4C calibration curve, rather than as a direct dating tool, apart from a time marker using bomb peak of "1"4C. Some possible causes for the relationship between speleothem "1"4C content (or dead carbon fraction: DCF) and karst hydrology have been proposed, such as changes in temperature, precipitation, drip water flow dynamics, cave air ventilation, soil air pCO_2. In this study, we investigated seasonal variation in "1"4C and δ"1"3C of drip water in Ryugashi Cave, Shizuoka Prefecture, central Japan, to examine the causes of the "1"4C and δ"1"3C variations in a speleothem. The results show that different "1"4C concentrations and δ"1"3C values of drip water from the Ryugashi Cave, were exhibited at different sites of the Caves No. 1, No. 3, and No. 4, which have different temperature, air pCO_2, and flow paths. Further, the "1"4C and δ"1"3C of drip waters showed seasonal variations at all sites, which were lower in fall and winter, and higher in spring and summer, though the extent of the variations was different among the sites. The "1"4C in drip waters tended to be correlated with the drip rates: "1"4C tended to be higher in drip waters with higher drip rates, and also correlated with rainfall amount around the Ryugashi Cave, especially for the drip waters in Cave No. 3, which are considered to have simpler flow paths. The increase in rainfall amount could bring the increase in drip rate of drip water, and then the decrease in interaction between solution and karst, resulting in "1"4C increase (DCF decrease) in drip water. Accordingly, the reconstruction of precipitation could be performed using "1"4C variation in a speleothem formed by drip water with simple flow dynamics.