Moisture content is the most important factor affecting the stability of slopes against heavy rainfall events. In most of risk management methods for slope disasters, the rainfall intensity is used as the dominant index to evaluate the probability of failure events in an area. However, each slope should have individual hydraulic characteristics, and its probability of failure should be different from other slopes even under the same rainfall conditions. Some slope may contain a lot of water quickly after starting of rainfall, while some slope may show quick drainage after it stopped to rain. Such individual properties of each slope can be evaluated by observing the time histories of moisture contents in the slope ground together with rainfall records. In this paper, an attempt to establish a mathematical model on drainage process in a slope ground is reported. There is a simple relationship between the current moisture contents and the drainage rate of moisture, although this relation is affected by the adjacent rainfall intensity. The drainage properties of a slope can be evaluated by watching the behaviors of moisture contents at rainfall events with relatively low intensity. And the model parameters obtained by such weak rainfall events can be used to estimate the drainage rate after heavy rainfall events with some correction of the parameters, by using real time monitoring system.