An information processing system (9) includes a sensor device (2) that is used for a left leg, a sensor device (3) that is used for a right leg, and an information processing apparatus (1) that processes data acquired by the sensor devices. The information processing apparatus includes a motion-candidate extracting unit (12) that extracts a point at which a motion of rotation of a leg about an axis in a direction of gravity occurs in the data acquired by the sensor devices, as a motion candidate of a direction change； a direction-change-motion determining unit (16) that determines a plurality of extracted motion candidates as a direction change motion when directions of the rotation are identical, and a motion candidate of the left leg and a motion candidate of the right leg alternately appear chronically successively, as a direction change motion； and a direction-change-feature extracting unit (17) that extracts a feature of the direction change motion that is determined by the direction-change-motion determining unit, by using the data acquired by the sensor devices, thereby enabled to extract a feature amount of a direction change motion highly accurately by using only sensors on both legs.
