Hydrate crystals maintain their structure when water is taken into the crystal, but the crystal structure changes or becomes amorphous when the crystal water is lost. There is a correlation between crystal structure and physical properties, and it is important to understand the phase transition behavior due to changes in humidity when controlling the quality of hydrates. Measuring the weight reduction of the sample while heating the hydrate by TG-DTA, the amount of desorbed water can be determined. In addition, if the hydrate is heated and the X-ray diffraction is measured by XRD-DSC, the dehydration temperature and the state after dehydration can be determined. Here, we investigated the dehydration behavior of a hydrated drug substance using TG-DTA and XRD-DSC.