This paper explains groundwater investigating methods, introducing particularly two methods. The resistivity tomography is a method to measure potential distribution generated when current is applied around an area to be investigated. Using the measurement result as projection data, the resistivity distribution is restructured within the area of the investigation. The paper summarizes the measurement method (measuring equipment, electrode arrangement, etc.), the analytic method (establishment of an initial model, and the alpha-center method to calculate theoretical potentials corresponding to the model), and an application example of the method in which the resistivity change was observed using permeation of salt tracers. The resistivity imaging method (two-dimensional resistivity analysis method) is superior in practicability to the above method under the present state of the art. This method does not differ from the conventional resistivity method in methodology, but improves the analytic accuracy over the conventional electrical investigation method as a result of analysis combining the simplified two-dimensional analysis method with the alpha-center method. A summary is given on the measuring and analyzing methods. 13 refs., 10 figs.