A method for assessing aging of erythrocytes that includes preparing an erythrocyte suspension from a blood sample; a allowing the erythrocyte suspension to pass through a first filter, separating aged erythrocytes that do not pass through the first filter, and non-aged erythrocytes that pass through the first filter, and to calculate a deformability of erythrocytes, allowing the separated non-aged erythrocyte suspension to pass through a second filter having micropores which diameter is smaller than the first filter, separating mild aged erythrocytes that do not pass through the second filter and juvenile erythrocytes that pass through the second filter, and to calculate a deformability of the non-aged erythrocytes; and assessing aging of erythrocytes, using the deformability of erythrocyte and the deformability of non-aged erythrocyte.