A non-invasive blood constituents measuring instrument measures blood constituent values including blood glucose concentration in a living body non-invasively. The instrument is composed of a light source 11 to irradiate a light containing plural wavelengths to a living body 13, a light receiver 14 to detect the light transmitted through a living body or reflected and scattered thereon, a spectrum analyzer 15 to analyze the light transmitted through the living body or reflected thereon at different times, a subtraction processor 18 to generate spectrum subtraction from the spectrum of the light at the different times measured by the spectrum analyzer 15, and a blood glucose concentration predictor 21 into which the output data of the subtraction processor 18 are input and which outputs a blood constituent value.