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This study described swimmers’ leg motions and the three-dimensional flow field around their legs during undulatory underwater swimming (UUS). We used the particle image velocimetry (PIV) method and a three-dimensional motion capture system. Seven male swimmers participated and we acquired EMG data of one male swimmer during UUS after the previous experiment. After the downward kick motion that includes the legs’ lateral rotation and the ankles’ dorsal flexion, water’s backward momentum was observed. During the upward kick motion, backward flow velocity decreased, but flow occurred in a vertical, upward direction. This suggested that UUS could demonstrate great propulsion power by generating jet flow through the downward kick motion that might be assisted by upstream flow from the upward kick motion.