PURPOSE: To miniaturize a case by arranging a lock drive mechanism and a restraint drive mechanism with a locking gear mechanism between them, and installing a restraining lever mechanism on the locking gear mechanism.

CONSTITUTION: A screw shaft 82 is freely rotatably fitted into a base 9 and supported against supports 93, 91 formed in the base 9, and is connected to the rotating shaft of a motor 81 so that it rotates integrally with the shaft. A nut member 83 is fitted around the screw shaft 82 so as to move in the direction of its axis as the screw shaft 82 is rotated. A projection 51f formed in a case main body 51 in such a way as to be offset from a wheel gear 63, is fitted into a recess 94 formed in the base 9 in such a way as to be offset from the wheel gear 63, so as to hold the base 9 against the case main body 51, so as to locate the screw shaft 82 between the wheel gear 63 and a cover 52 and opposite a worm gear 62 with the wheel gear 63 between them. Since the motor 81 and the screw shaft 82 are unitized by the base 9, the wheel gear 63 of a locking gear mechanism 6B and the screw shaft 82 of a restraint drive mechanism 8A can be arranged to overlap each other.