PROBLEM TO BE SOLVED: To provide a method of determining a concentration of microalgae or metabolites nondestructively comprised in a culture solution.SOLUTION: A concentration deciding method of microalgae or metabolites related to the invention comprises an irradiation step of irradiating a solution of microalgae with a light of a light source for measurement having a given center wavelength and wavelength band region which represent a feature of microalgae or metabolites a measurement step of measuring a light intensity of the diffuse light of the solution of the microalgae and a determination step of determining a concentration of the microalgae or the metabolites based on the light intensity of the light measured in the measurement step.
