As for the groundwater in the area of landslide, to know the source of it, the place to where the groundwater flows in and the distribution of the groundwater vein lead to the well understanding of the effect of groundwater on the landslide activities and subsequently to the reasonable planning of drainage works. In this article, authors deal with how the groundwater vein is inferred in three crystalline schist landslides in Shikoku, Japan. Among chemical indices of groundwater quality, two major anions HCO₃^- ion and SO₄^2- ion were proved to be significant. Weathering and/or weakening of whole sliding body depends on HCO₃^- ion, while SO₄^2- ion affects strongly to the weathering of pelitic schist, which is closely connected to the formation of the slip surface. From the point view of hydrogeology, the geological structure of the slide zone restricts fairly the development of underground water vein. Using only the solved ingredients for the purpose of detecting groundwater vein is not necessarily sufficient as a methodology, authors have confirmed that the presented way could attain a certain level of research.