Abstract: This paper deals with the modellings of temporal changes in earth resistivity (i.e. apparent resistivity) on the surface of the earth with various electrical structure parameters and various configuration parameters (seperation of electrods), caused by some temporal changes in the true resistivity of different kinds and in different locations inside the earth in terms of the mathematical formula of apparent resistivity for a multilayer model. The relations bet-tween the observed temporal change in apparent resistivity on the earth surface and the temporal changes in the true resistivity existing inside the media have also been discussed. This suggests that in order to solve the problem of discriminating the anomlous changes from inter-ferrence influences, the observational system with single seperation between electrodes currently in operation at most monitoring stations in China should be changed into that with multi--separations, and the technique of inverse for temporal changes in apparent resistivity should be developed to realize the transition from earthquake prediction through guess work to prediction based on physical principles in the study of earthquake prediction.