A seating condition detecting system has four load sensors, a belt tension sensor and a self-diagnostic system. The first and second abnormal ranges are set for detecting abnormal operations of the system. The upper limit of the first range is set so that a total load becomes heavier as a belt tension becomes higher. The lower limit of the second range is set so that the total load becomes heavier as the belt tension deviates from the point at which the total load becomes lightest. When the total load falls in the first range or the second range, an abnormal operation of the seating condition detecting system is determined.