Hemoglobin in a sample solution is quickly and reliably denatured; at the same time, quick and accurate measurement of hemoglobin and a hemoglobin derivative is realized. In a method for measuring hemoglobin and a hemoglobin derivative, and a reagent composition, a measurement kit, an analysis device, and an analysis system used in the method, a sample solution containing a blood component is treated with a nonionic surfactant, an oxidizing agent, and a metal salt to denature hemoglobin in the sample solution to measure the hemoglobin, and thereafter the amount of a hemoglobin derivative in the sample is measured by an immunological method using an antibody specifically binding to a denatured site of the denatured hemoglobin derivative.