A small and portable instrument for measuring blood sugar level of a human body noninvasively with no error comprising a light source control section for irradiating the measurement part of a finger with irradiation lights in two different near-infrared wavelength regions, photodetectors for receiving lights and of the irradiation lights quantities of transmitted lights. A relative transmittance, i.e. the ratio of the quantities of transmitted lights of the same wavelength detected at the two positions, is calculated for each wavelength and the blood sugar level is determined using the relative transmittance of each wavelength.