In a normal walking motion, a rightward acceleration Ax becomes dominant in a left foot landing period, and a leftward acceleration Ax becomes dominant in a right foot landing period. Thus, an integrated value S of the acceleration Ax over an intermediate period between two consecutive impact generation timings changes in a regular pattern of alternately increasing and decreasing with respect to each step. Accordingly, by comparing the integrated values S acquired with respect to two consecutive intermediate periods temporally separated by one impact generation timing, it is possible to determine whether the one impact generation timing corresponds to the landing timing of the left foot or the landing timing of the right foot.
