A crystal form determination method includes a first process of preparing a measurement target object; a second process of entering a terahertz wave to the measurement target object and detecting the terahertz wave from the measurement target object to acquire a plurality of detection results corresponding to a plurality of times separated from each other; and a third process of determining a type of crystal form contained in the measurement target object. In the third process, when a frequency characteristic having a third peak at a third frequency different from a first frequency related to a first peak corresponding to an anhydride and a second frequency related to a second peak corresponding to a hydrate exists among the plurality of frequency characteristics, determination is made that the crystal form different from the anhydride and the hydrate is contained in the measurement target object.