A sample containing hemoglobin A1c (HbA1c) is simultaneously brought in contact with a solid phase and anti-HbA1c antibody, the anti-HbA1c antibody bound to the adsorbed HbA1c on the solid phase and the anti-HbA1c antibody in solution are separated, and the anti-HbA1c antibody bound to the adsorbed HbA1c on the solid phase is detected. HbA1c % is determined by one-step immunoassay, and the time required for measurement is shortened in comparison with conventional immunoassay methods. Since the exposure time to the pretreatment solution present in the reaction solution during the immune reaction is shortened, formation of a precipitate in the sample as well as inactivation of the enzyme of the enzyme-labeled anti-HbA1c antibody, when used, is reduced.