An optical rotation angle of linearly polarized light is modulated to obtain modulated light. The modulated light is passed through an inspection object containing an optically active substance. The optically active substance rotates an optical rotation angle of the modulated light. An intensity of light coming out of the inspection object is measured. Variation in the optical rotation angle of the modulated light is calculated based on the measured intensity and the angle to which the linearly polarized light is modulated. A concentration of the optically active substance in the pulsing component of the inspection object is calculated based on the variation in the optical rotation angle. Thus, concentration of glucose in the blood can be calculated without harming the patient.