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The flow field around a swimmer has been assumed to be steady or quasi-steady in the estimation of flow force acting on hand/arm in spite of its extremely unsteady characters, because the convenient means for measuring the flow field has been lacking. A sophisticated technique called the PIV (Particle Image Velocimetry) was developed and enabled us to estimate the unsteady flow field. The PIV technique can visualize the unsteady flow field and can detect the vortex behavior around a hand. On the other hand, motion analysis has been widely used in the field of biomechanics of sport. This method can also be expected to evaluate the swimmer's complicated motion. Our study is to clarify the relationship between the vortex behavior and the motion of a hand in crawl swimming. Two methods were combined to know the generating mechanism of swimming thrust. Measurements were made for two subjects ; one is a female Olympic swimmer (subject 1) and the other a male with no competitive career (subject2). The trajectory of the plam of the subject 1 was S-shaped motion. From the change of the palm inclination angle, it was suggested that the palm of the subject 1 reversed the orientation of the circulation. On the other hand, the palm of subject 2 did not reverse the orientation of the circulation. It is plausible that subject 2 does not generate any strong and coherent vortices. From the results of visualization of flow field, subject 1 generates strong vortices or vortex pair after the phase turned from in-sweep to out-sweep. In addition, the shed vortex pair follows a jet flow in the direction of the flume flow. In contrast, subject 2 did not shed any vortex pairs. It was confirmed that the hand motion in swimming was closely related to the vortex generation and also to the thrust.