A novel method for efficiently estimating landslide behaviors by monitoring the groundwater composition has been examined. Basically, the ion-exchange reaction between the groundwater and soil particles continuously takes place. The reaction rate naturally depends on the area where the reaction undergoes. Accordingly, it is expected that the larger the surface area of the soil particles in the ground, the faster the ion-exchange reaction would apparently be. Therefore, the groundwater composition should be changed before the ground distortion becomes much larger. This phenomenon was successfully observed in fact in some landslides. The useful information on the groundwater composition, which is to be offered to estimate the next occurrence of landslide, is reported, and its meaning is also discussed.