Abstract: A method and apparatus for measuring blood glucose concentration by irradiating blood vessels with electromagnetic radiation, where the method and apparatus uses near-infrared radiation diffuse-reflection laser spectroscopy. This invention uses electromagnetic radiation of a wavelength that is transmitted through the skin to the measurement region, for example, a blood vessel. Since skin is mostly composed of water (H.sub.2 O), which absorbs infrared radiation in nearly the entire infrared spectral range, only radiation from a certain, narrow portion of the infrared spectral range called the "water transmission window" is transmitted through the skin. The present invention uses electromagnetic radiation with a wavelength of 1.3 .mu.m- 1.8 .mu.m radiation from a semiconductor diode laser. When electromagnetic radiation of these wavelengths irradiates the skin, light is transmitted through the skin to the blood vessel where the light interacts with the heterogeneous components of the blood.