© 2019 IEEE. To increase the productivity of crops, growth methods based on direct monitoring of physiological information in plants is required. In this study, we proposed a novel micro sensor device for simultaneously measuring moisture and nutrient substance dynamics in plant xylems. A nutrient substance dynamics sensor comprises an electrical conductivity (EC) sensor for measuring nutrient substance concentration and a resistance temperature detector (RTD) for temperature compensation of the EC value. To obtain a high-performance EC sensor, we fabricated a three-dimensional (3D) electrode on a needle-shaped Si probe based on micro-electromechanical system technology. We succeeded in fabricating an EC sensor equipped with a 3D electrode (cell constant of approximately 1200 m-1) and an RTD and demonstrated the feasibility of measuring the nutrient substance dynamics in plants.