An erythrocyte has high deformability. In the shear field, it deforms from biconcave disc to ellipsoid, and make tank-treading motion at the membrane. When the membrane is ruptured by fatigue, contents get out from the inside of the cell (hemolysis). At the fatigue test of the membrane in the shear field, “the shear stress” as “the amplitude” and “the shear rate” × “the exposure time” as “repeat count” are critical parameters. For the quantitative fatigue test, the uniform shear field has been realized between the rotating concave cone and the stationary convex cone. With the rheoscope, deformability is evaluated with shear stress responsiveness and with critical deformation calculated from an exponential curve between the deformation ratio (the ratio between the major axis and the minor axis of the ellipsoidal shape) and the shear stress. Deformability decreases at erythrocytes of high density after shearing. The erythrocytes deformation ratio varies periodically at the double frequency of tank-treading motion of the membrane, when the erythrocyte has the sublethal damage point on the membrane.