Since March 1977 we have measured Cl- concentration in the groundwater almost every day at Shiota mineral spring near the Yamasaki fault, which is situated in the northern Kinki and Chugoku Districts, Southwest Japan, to see if the Cl- concentration shows any variations which may be related to seismic activity near Shiota mineral spring. From March 1977 through December 1984 there occurred 6 earthquakes of M3.5 or greater near Shiota mineral spring and two of M6.0 or greater in the northern Kinki and Chugoku Districts. The Cl- concentration anomalously changed four times before these earthquakes though the pattern of preseismic changes of the Cl- concentration was not uniform. We conclude that measurement of Cl- concentration at a carefully chosen mineral spring near an active fault should give information useful for earthquake prediction.