A simple calibration method for calibrating an instrument for measuring a biogenic substance, using near-infrared spectral spectroscopy is realized. The calibration method comprises the step 1 of measuring a specific substance of a biological object with the use of an instrument (1) for measuring a biogenic substance, using a confocal optical system (2), the step 2 of using the instrument (1) for measuring a biogenic substance, using near-infrared spectral spectroscopy, thereby measuring a specific substance in the same region of the biological object, the step 3 of comparing a measured value of the specific substance, measured in the step 1 with a measured value of the specific substance, measured in the step 2 and the step 4 of executing an operation in the step 3 at least once after the elapse of predetermined time.
