To fit a seat belt on an occupant without looseness and to sufficiently reduce oppressive sensation against the occupant by controlling energizing force by moving a movable member by a supersonic wave motor so that the energizing force becomes a target value.

Drawing quantity of a seat belt is detected as rotational quantity of a reel 20 by a rotary encoder 107. When a tong is installed on a buckle and a buckle switch is switched on, a supersonic wave motor 24 rotates a spring case 34 by specified times in the winding-up direction of a spring 32, and winding force of the spring 32 is improved. Consequently, looseness of the seat belt is removed, and the seat belt is closely fitted to an occupant. As oppressive sensation to the occupant is strong as it is, the spring case 34 is rotated in the loosening direction of the spring 32 by specified times. Accordingly, the oppressive sensation of the seat belt is reduced or eliminated.