The blood sugar value estimation apparatus is configured to non-invasively calculate the blood sugar value with time on the basis of the optical spectrum of the living body measured with time and the calibration model. The apparatus comprises a calibration model creating means configured to create a calibration model from the calibration models or a calibration model from a plurality of the datasets for creating the calibration model. The apparatus is configured to measure a bio-spectrum of a person being tested to set the reference spectrum, and to measure a difference spectrum of a difference between the reference spectrum and a measurement spectrum measured at a time other than a time when the reference spectrum is measured, and to change the calibration model for calculating according to the variation of the difference spectrum. Consequently, the blood sugar value is estimated, especially monitored with time, with high-accuracy.