A method of assaying a glycated protein in a sample with the use of redox reaction, in which highly reliable measurement can be obtained. A sample containing a glycated protein is treated with protease in the presence of a sulfonic acid compound, so that the glycated protein is degraded. The glycated portion of the resultant glycated protein degradation product is reacted with fructosyl amino acid oxidase, and this redox reaction is measured, thereby determining the amount of glycated protein.