In order to calculate and specify a valley wavelength easily and in a short period of time, a detection device for intermolecular interaction is disclosed that is equipped with a detector provided with a ligand, a white light source that irradiates the detector with white light, a spectrometer that detects the light reflected from the detector, and a control device that controls the white-light source and the spectrometer, wherein the aforementioned control device obtains a reflection spectrum by calculating the reflectivity over a fixed wavelength interval, approximates the aforementioned reflection spectrum as a high-dimensional function, selects a wavelength interval comprising the minimum reflectivity from the aforementioned high-dimensional function, approximates the aforementioned high-dimensional function with the aforementioned wavelength interval as a quadratic function of a lower order, and obtains a solution by which the aforementioned quadratic function is differentiated with respect to the wavelength and the value thereof becomes 0.