A center-of-gravity detecting system comprising: a motion detector for detecting a vertical motion of a travelling object in a self-weight direction of the travelling object during travel, and a horizontal motion of the travelling object in a widthwise direction of the travelling object during travel; and an arithmetic unit; wherein the arithmetic unit is adapted to obtain a frequency of the vertical motion in the self-weight direction and a frequency of the horizontal motion in the widthwise direction corresponding to a width of the traveling object, by using the motion detector; the arithmetic unit is adapted to calculate a center-of-gravity location of the travelling object in the self-weight direction in a cross-section perpendicular to the travel direction and a center-of-gravity location of the travelling object in the widthwise direction in the cross-section perpendicular to the travel direction, using the frequency of the vertical motion in the self-weight direction, the frequency of the horizontal motion in the widthwise direction, a central angle of the horizontal motion in the widthwise direction, and the width of the traveling object; characterised in that the arithmetic unit is adapted to derive coefficients in a formula in which a relation between the width of the travelling object and the frequency of the horizontal motion in the widthwise direction is formulated, using the center-of-gravity location in the self-weight direction, the center-of-gravity in the widthwise direction, the frequency of the vertical motion in the self-weight direction, and the central angle of the horizontal motion in the widthwise direction; the arithmetic unit is adapted to calculate a frequency of the horizontal motion in the travel direction, using the derived coefficients in the said formula, where the width of the travelling object is substituted by the length of the travelling object in the travel direction, in the said formula; and the arithmetic unit is adapted to calculate a center-of-gravity location of the travelling object in the travel direction, using the frequency of the vertical motion in the self-weight direction, a frequency of the horizontal motion in the travel direction, the center-of-gravity location in the self-weight direction, and the length of the travelling object in the travel direction. a motion detector for detecting a vertical motion of a travelling object in a self-weight direction of the travelling object during travel, and a horizontal motion of the travelling object in a widthwise direction of the travelling object during travel; and an arithmetic unit; wherein the arithmetic unit is adapted to obtain a frequency of the vertical motion in the self-weight direction and a frequency of the horizontal motion in the widthwise direction corresponding to a width of the traveling object, by using the motion detector; the arithmetic unit is adapted to calculate a center-of-gravity location of the travelling object in the self-weight direction in a cross-section perpendicular to the travel direction and a center-of-gravity location of the travelling object in the widthwise direction in the cross-section perpendicular to the travel direction, using the frequency of the vertical motion in the self-weight direction, the frequency of the horizontal motion in the widthwise direction, a central angle of the horizontal motion in the widthwise direction, and the width of the traveling object; characterised in that the arithmetic unit is adapted to derive coefficients in a formula in which a relation between the width of the travelling object and the frequency of the horizontal motion in the widthwise direction is formulated, using the center-of-gravity location in the self-weight direction, the center-of-gravity in the widthwise direction, the frequency of the vertical motion in the self-weight direction, and the central angle of the horizontal motion in the widthwise direction; the arithmetic unit is adapted to calculate a frequency of the horizontal motion in the travel direction, using the derived coefficients in the said formula, where the width of the travelling object is substituted by the length of the travelling object in the travel direction, in the said formula; and the arithmetic unit is adapted to calculate a center-of-gravity location of the travelling object in the travel direction, using the frequency of the vertical motion in the self-weight direction, a frequency of the horizontal motion in the travel direction, the center-of-gravity location in the self-weight direction, and the length of the travelling object in the travel direction.