To minimize the damage caused by slope failure, knowledge of the increase in water content in mountain soil is important; however the current technology for monitoring this is insufficient. In this study, an EC sensor chip with an insulation layer with increased thickness, designed to reduce leakage current, was fabricated. Utilizing the chip, a wireless sensor terminal was constructed for long term continuous monitoring of the water content in mountain soil. The usefulness of the EC sensor for predicting slope failure was confirmed.