A spectrometer equipped with an integrating sphere is used to measure a reflection spectrum from a suspension in a container when the suspension is irradiated with measurement light, of which wavelengths are selected from a wavelength range including near infrared. Reflection spectra of a number of types of standard samples, of which concentrations in the suspension are already known, are used to prepare a measurement model in accordance with an assay technique on the basis of a recursion. A concentration of the material in the suspension is found using the measurement model.