The experimental methodology has been designed for evaluation of the cyclic deformation of an erythrocyte in the pulsatile shear field in vitro. To observe the deformation of the suspended erythrocytes in the Couette type of the shear flow, a rheoscope system has been manufactured. The system consists of a pair of counter-rotating parallel disks and an inverted phase-contrast microscope. The human erythrocytes were suspended in the dextran aqueous solution of high viscosity. The rotating speed varies sinusoidally to make the pulsatile shear field. The deformation of each erythrocyte was measured at the video image of the rheoscope. The experimental results show that the system is available to measure the following behavior of an erythrocyte. The ellipsoidal shape of each erythrocyte varies cyclically to follow the pulsatile cyclic shear field. The delay of deformation phase of each erythrocyte in the cycle is related to the frequency of the cyclic variation of the shear field. The delay is also related to deformability of the erythrocyte.