An erythrocyte can be deformed very easily by even very small amounts of stress, enabling it to flow smoothly through approximately 3 µm diameter microvessels despite its larger size of 8 µm in diameter and 2 µm in thickness. Reduction of erythrocyte deformability leads to poor circulation particularly in microvessels, which may result in hypertension and various circulatory diseases. To clarify blood microcirculatory properties, it is very important to understand the deformability of erythrocytes. However, a standard method for quantitative evaluation of erythrocyte deformability has not yet been established. In this study, an apparent Young's modulus and a time constant for the recovery of erythrocyte shape following uniaxial stretching are proposed as deformability indices. When an erythrocyte was pulled horizontally on opposite ends using two micropipettes, tensile stress and strain were measured, and an apparent Young's modulus was calculated based on Hooke's law. On returning to its initial shape after the erythrocyte was released from stretching, the elapsed time and strain were measured, and a time constant for shape recovery was calculated based on the Kelvin model. The average apparent Young's modulus was 16.0Pa, and the average time constant was 113.4ms, with a slight tendency toward an inverse relation between the two physical quantities.