Abstract: A method for quantifying a glucose concentration is a method for quantifying a glucose concentration in which near-infrared light is emitted onto a living organism and a glucose concentration in a biological tissue is measured using a signal obtained by receiving diffusely reflected light or transmitted light from the biological tissue. A concentration calculation step calculates a concentration index of a glucose component by using at least a spectrum of a water component, a spectrum of a glucose component, and a spectrum of a fat component to synthesize a difference spectrum between a measurement spectrum at a time of measurement of a glucose concentration and a spectrum serving as a reference obtained previous to the measurement spectrum. A glucose concentration calculation step calculates a glucose concentration in the living organism using the calculated concentration index.