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