The present invention provides easy and stable measurement of the concentration of a specific component contained in a living body, even when a fluid such as water, saliva or sweat is present between an optical element and the living body or when the measurement is taken at a plurality of measuring parts of a living body. In the measurement of the present invention, a calibration line to be used to correct the influence of a change of a fluid layer present between a living body and an optical element is determined, a measured value is corrected based on the calibration line and the concentration is obtained from the corrected value.