The objective of the present invention is to provide a method and instrument for measuring glucose concentration in blood using infrared spectroscopy. Glucose concentration in blood is measured based on an integrated value obtained by measuring an absorption spectrum that includes the wavenumber range of 1020-1040 cm−1, and by integrating the intensity of absorption of the wavenumber range of 1020-1040 cm−1 within the absorption spectrum. Alternatively, a glucose concentration in blood is measured based on an integrated value through measuring an absorption spectrum including the wavenumber range of 1010-1050 cm31 1; obtaining a second-derivative spectrum by calculating a second derivative of the absorption spectrum that includes the wavenumber range of 1010-1050 cm−1 in the absorption spectrum; determining an integration range based on the second derivative spectrum; and then obtaining the integrated value by integrating the intensity of absorption based on this determined integration range.