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In a car crash, if deformation of occupant thorax is held to the allowable value, no aftereffect will remain. In order to realize this condition, a seat belt tension must be adjusted instantaneously by a feedback control system considering the thorax deformation. Therefore, the seat belt tension control system on the basis of the semi-active shock control has been proposed. An advantage of this system is requiring no large power actuator because the seat belt tension can be controlled by a brake mechanism. And the effectiveness has been confirmed by an experiment using the scale model of about 1/10. Though the deformation of the thorax model was made to settle at the desired value, the overshoot of about 20% was occurred for the narrow control range of the actuator. In this study, the control law and the actuator which controls the tension using the friction are improved in order to suppress the overshoot of the deformation, and the effectiveness is confirmed by the control experiment. As the result, the overshoot of the deformation can be suppressed to 10%, the deformation after the overshoot is made to settle near the target.