The deformation of an erythrocyte has been observed microscopically in the shear flow to detect the sublethal damage of an erythrocyte in vitro. A rheoscope system has been manufactured to observe the deformation of the suspended erythrocytes in the shear flow. The rheoscope consists of a pair of parallel disks and an inverted phase-contrast microscope. The human erythrocytes were suspended in the dextran aqueous solution, which has high viscosity. The erythrocytes are sheared in the Couette flow between a pair of counter rotating disks. The experiments with the rheoscope show following results. The erythrocytes deform from a biconcave to an ellipsoidal shape. The erythrocytes deform periodically at the double frequency of tank tread motion of the membrane of the deformed erythrocyte, when erythrocytes have the sublethal damage on the membrane.