There is provided a method for specifically determining a glycated .beta.-chain N-terminal of glycated hemoglobin using enzymes without a separation operation, and a determination reagent kit therefor. A protease that cleaves a glycated amino acid and/or a glycated peptide from a glycated .beta.-chain N-terminal without substantially cleaving a glycated amino acid or a glycated peptide from a glycated .alpha.-chain N-terminal of glycated hemoglobin or a fragment thereof is screened. The method of specifically determining a glycated .beta.-chain N-terminal of glycated hemoglobin and the determination reagent kit are provided by using the protease obtained by the screening method. According to the present invention, a glycated .beta.-chain N-terminal of glycated hemoglobin can specifically be determined without a separation operation.