Precipitation-induced landslide is one of the most important geo-hazards that claim many casualties every year world-side. To mitigate this hazard, the present text addresses monitoring and early warning in relatively small and shallow slope failures that occur at thousands of places in the world. Because they occur suddenly in slopes that have been intact for many decades, people do not fully recognizes the existing risk until the last moment, leading to damage to lives and properties. The authors have developed an inexpensive instrument for early warning that enables people evacuate in advance. Results of recent validation studies are going to be introduced in this article.