An earth flow occurred on 12th to 13th of July, 1972, in more than 3,000 locations in Obara-mura (area: 75km²), Nishimikawa of Aichi Prefecture. Water samples were taken from the earth flow area and analyzed for Na⁺, Ca²⁺, Mg²⁺, Cl^-, HCO₃^-, SO₄^2- and soluble SiO₂. The concentrations of Ca²⁺+Mg²⁺ and HCO₃^- were varied greatly from location to location, indicating the difference of the dissolution of Ca²⁺+Mg²⁺ in waters from clay, sand and rock materials in the different areas. The chemical compositions of the waters indicate that the earth flow occurred only owing to a very heavy rain, and that in some locations a chemical weathering is progressing rather actively to form clay materials under the ground through the interaction of carbonated water with rock materials.