Abstract: To intensity-modulate laser light periodically wavelength-modulated by and emitted from a wavelength-variable semiconductor laser 11. To separate the laser light into optical paths 13a, 13b with a beam splitter 14 to irradiate an examined location 17 for assessing blood sugar through path 13a. To detect the intensity of transmitted or reflected light from examined location 17 with a first detector 21 and the intensity of laser light passing through path 13b with a second detector 22 to detect the ratio of the former intensity to the latter intensity with a logarithmic ratio amplifier 25. To detect the rate of change in the ratio with respect to the change in wavelength of the wavelength modulation with a lock-in amplifier 26 to obtain a derivative spectral signal of the absorption spectrum of glucose. An arithmetic processor 27 detects blood sugar in the examined location from the derivative spectrum.