A dendrometer is a sensor that measures the amount and speed of tree trunk growth. It can be divided into two types: the visual confirmation type such as the measuring type and the electric type manufactured by our company. The electrical type can output some kind of electrical signal such as voltage or resistance, so it can automatically record data at arbitrary time intervals, and at the same time, it is characterized by high resolution. Since the electric method can obtain a resolution approximately 1/1000 that of the measure method, it is possible to obtain correlations not only with the amount of growth but also with time changes (from several minutes intervals) such as water potential and sap flow, which are important parameters.