Abstract: A concentration determination apparatus may include: an irradiation unit that irradiates short-pulsed light to an observed object; a light receiving unit that receives light backscattered from the observed object; a light intensity acquisition unit that acquires an intensity distribution of the light; an optical path length distribution storage unit that stores a model of an optical path length distribution in each layer of the plurality of layers of the short-pulsed light; an optical path length acquisition unit that acquires the optical path length distribution of each layer; a time-resolved waveform storage unit that stores a model of a time-resolved waveform; a light intensity model acquisition unit that acquires a light intensity model; an integral interval calculation unit that calculates a time range of an area; an optical absorption coefficient calculation and acquisition unit that calculates and acquires an optical absorption coefficient; and a concentration calculation unit that calculates the conce