A center-of-gravity detecting system includes a motion detector for detecting a vertical motion of a travelling object and a horizontal motion of the travelling object, and an arithmetic unit that calculates a center-of-gravity location of the travelling object using the frequency of a vertical motion of the travelling object, a frequency of the horizontal motion of the travelling object, a central angle of the horizontal motion of the travelling object, and the width of the traveling object. The arithmetic unit calculates a center-of-gravity location, using the frequency of the vertical motion, a frequency of the horizontal motion, the center-of-gravity location, and the length of the travelling object in the travel direction.