Abstract: A hygroscopicity evaluation method includes: a first step of preparing a first sample and a second sample; a second step of acquiring a first detection result for the first sample and a second detection result for the second sample by making a terahertz wave incident on each of the first and second samples; and a third step of evaluating the hygroscopicity of a measurement target object based on a first frequency characteristic calculated from the first detection result and a second frequency characteristic calculated from the second detection result. In the third step, the magnitude of the hygroscopicity of the measurement target object is evaluated based on the difference between the magnitude of a first peak of the first frequency characteristic in a reference frequency range and the magnitude of a second peak of the second frequency characteristic in the reference frequency range.