a method for quantifying glycated hemoglobin contained in a sample , the method including : a process for mixing the sample with a protease in the presence of a composition of a cationic surfactant and a tetrazolium salt to obtain an aqueous glycated peptide solution containing a glycated peptide ; a process for mixing the aqueous glycated peptide solution obtained in process with a fructosyl peptide oxidase to obtain hydrogen peroxide , the aqueous glycated peptide solution here containing the composition ; and a process for calculating the concentration of the glycated hemoglobin on the basis of the amount of hydrogen peroxide solution obtained in process .