The concentration of substance in blood is measured non-invasively, with high accuracy and with simple configuration. Laser light 100 generated by a light source 10 is locally irradiated on the body epithelium F of a subject, and the resulting diffused reflected light 200 is detected by a light detector 40. The laser light 100 has a wavelength of 9.26 µm. The laser light 100 is generated by converting and amplifying pulsed excitation light 101 from an excitation light source 11 to a long wavelength. A plate-shaped window 300 that is transparent to mid-infrared light is brought in close contact with the body epithelium F. The glucose concentration in interstitial fluid can be calculated using normalized light intensity calculated from a signal ratio of signals from a monitoring light detector 16 and light detector 40.
