The present invention relates to a method for quantitative determination of an α-glycated peptide in a sample, comprising causing protease to act on a whole blood and/or blood cell sample, causing an elimination reagent containing one or a plurality of types of ketoamine oxidase to act on the resultant, eliminating an α-glycated amino acid, an ε-glycated amino acid, an ε-glycated peptide, or a combination thereof, and then determining the α-glycated peptide in the sample using oxidase that acts on the α-glycated peptide. The present invention also relates to an elimination reagent and a kit to be used for such method. According to the present invention, measurement errors in quantitative determination of a glycated protein such as glycated hemoglobin can be reduced, and thus a precise measured value can be obtained.