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 sensor for multipoint measurements in the soil. To improve contact characteristics between sensor surface and free water in soil, SiOx film of hydrophilic property covered on our conventional sensor chip. For contact property test, the sensor chip measured impedance of model soils. The proposed sensor achieved to stable contact the free water in the soil. Moreover, the chip operated for long period of time in mountain slope. The sensor could measure nearly theoretical outputs which responded to rainfalls. We succeeded to fabricate the stably soil monitor sensor with hydrophilic film SiOx.