An information processing system comprising: a first motion sensor that is used for a left leg of a user; a second motion sensor that is used for a right leg of the user, wherein the first motion sensor and the second motion sensor each comprises a gyro sensor configured to acquire data of an angular velocity around three axes that are perpendicular to each other; and an information processing unit configured to process the data acquired by the first motion sensor and the second motion sensor, wherein the information processing unit includes a motion-candidate extracting unit configured to extract a plurality of peaks in first data among the acquired data from the first motion sensor, and in second data among the acquired data from the second motion sensor, as motion candidates of a direction change, wherein the peaks indicate a rotation amount of the left leg or the right leg, respectively, in a direction transverse to a walking direction of the user, wherein the rotation amount of the left leg or the right leg indicated by the peaks is larger than a first threshold from a time at which one foot comes off a ground until a time of landing wherein, when more than one of said peaks successively appear in the first data or the second data in a predetermined time on the same leg, extract the more than one of said peaks as a single motion candidate of one step; a direction-change-motion determining unit configured to determine a motion indicated by the motion candidates extracted by the motion-candidate extracting unit as a direction change motion based on whether the first data and the second data indicate that a motion candidate of the left leg and a motion candidate of the right leg alternate in temporally appearing therein, and based on whether signs of the rotation amounts of the alternating motion candidates are the same, wherein the direction-change-motion determining unit is configured to determine the motion indicated by the motion candidates extracted by the motion-candidate extracting unit as the direction change motion when the signs of the rotation amounts of the alternating motion candidates are the same, and when the first data and the second data indicate that the motion candidate of the left leg and the motion candidate of the right leg alternate in temporally appearing therein; and a feature extracting unit configured to extract a feature of the direction change motion that is determined by the direction-change-motion determining unit, by using the data acquired by the first motion sensor and the second motion sensor. a first motion sensor that is used for a left leg of a user; a second motion sensor that is used for a right leg of the user, wherein the first motion sensor and the second motion sensor each comprises a gyro sensor configured to acquire data of an angular velocity around three axes that are perpendicular to each other; and an information processing unit configured to process the data acquired by the first motion sensor and the second motion sensor, wherein the information processing unit includes a motion-candidate extracting unit configured to extract a plurality of peaks in first data among the acquired data from the first motion sensor, and in second data among the acquired data from the second motion sensor, as motion candidates of a direction change, wherein the peaks indicate a rotation amount of the left leg or the right leg, respectively, in a direction transverse to a walking direction of the user, wherein the rotation amount of the left leg or the right leg indicated by the peaks is larger than a first threshold from a time at which one foot comes off a ground until a time of landing wherein, when more than one of said peaks successively appear in the first data or the second data in a predetermined time on the same leg, extract the more than one of said peaks as a single motion candidate of one step; a direction-change-motion determining unit configured to determine a motion indicated by the motion candidates extracted by the motion-candidate extracting unit as a direction change motion based on whether the first data and the second data indicate that a motion candidate of the left leg and a motion candidate of the right leg alternate in temporally appearing therein, and based on whether signs of the rotation amounts of the alternating motion candidates are the same, wherein the direction-change-motion determining unit is configured to determine the motion indicated by the motion candidates extracted by the motion-candidate extracting unit as the direction change motion when the signs of the rotation amounts of the alternating motion candidates are the same, and when the first data and the second data indicate that the motion candidate of the left leg and the motion candidate of the right leg alternate in temporally appearing therein; and a feature extracting unit configured to extract a feature of the direction change motion that is determined by the direction-change-motion determining unit, by using the data acquired by the first motion sensor and the second motion sensor.