Four-electrode-method, commonly known as Wenner method, for measuring specific resistance of the earth is reconsidered and developed not only theoretically but also practically. The method has such strong points mainly from hardware that measurement can be done in short period, electrodes do not interfere with the change of soil-water content, any measured space under the ground can be chosen by changing the interval of the electrodes, and sensoring elements are dependable when they are set under the ground. These strong points are favorable in the field measurement. Considering these strong points and through theoretical analysis, the "Normalized apparent specific resistance" is proposed to be measured in order to observe the dynamical change of soil-water content responding to the time-varying rain input at the surface of the ground. To establish fast measurement system, electronic circuit (ρmeter-III) is developed and used successfully in the experiment of rain infiltration.