To minimize the damage caused by slope failure, knowledge of the increase in water content in mountain soil is important. Our group has been studying miniaturized impedance sensors for multipoint measurements in soil. Conventional sensors with a SiNx top film sometimes cannot detect changes in the water content in soil resulting from rain. Experiments on the slide distances of water droplets were performed using SiOx and SiNx films. As the slide distance of the SiOx film was shorter than that of the SiNx film, it was confirmed that the SiOx film is more hydrophilic than the SiNx film. To improve contact characteristics between the sensor surface and free water in soil, we covered our conventional sensor chip with a hydrophilic SiOx film. As a contact property test, the sensor chip measured the impedances of model soils. The proposed sensor achieved stable contact with the free water in the soil. Moreover, the chip operated for a long period of time on a mountain slope. The sensor could measure nearly theoretical outputs in response to rainfall. Thus, we succeeded in fabricating a stable soil monitor sensor with a hydrophilic SiOx film.