For proper irrigation planning, a non-destructive and nonpenetrating method of measuring water content of plants and soils is needed. Near-infrared (NIR) reflectance technology is designed to provide rapid, non-destructive, and accurate measurements of water inside materials. A total of 40 reflectance spectra (1550–1950 nm) from different plant and soil samples were measured in the laboratory using an inexpensive hand-held sensor. The result of drying experiments showed that there were close relationships between spectrum parameters of the water percentage of leaves and soil. The result of log-linear model and linear regression analysis showed significant effects to spectrum parameters based on the water percentage of leaves and soil respectively. The result of our study confirmed that the absorption of NIR light by samples’ water was highest around 1940 nm and lowest at around 1650 nm. It is suggested that the simple index using single wavelength (1940 nm) can be used for universal index to estimate the water content of leaves. The ANCOVA analysis showed significant differences of regression among soil types in Japan. This result suggests that different calibration models for the water percentage of the soil is needed for each soil type. These results reveal that an inexpensive hand-held NIR sensor can be applied to measure water status of living plants in the field.