Patent ID: 8308099

Claim:
A seat belt retractor comprising: an acceleration sensor which detects an acceleration in a horizontal direction; and a lock mechanism which locks an extraction of a seat belt in accordance with the acceleration detected by the acceleration sensor, wherein the acceleration sensor comprises: a sensor case comprising a flat supporting surface which is constantly maintained horizontal; a self-standing inertia body having a substantially cylindrical shape with a flat bottom surface, which is placed on the supporting surface, the self-standing inertia body being installed inside the sensor case in a self-standing condition and tilting when the acceleration of a certain level or more in the horizontal direction is exerted; and an actuating member which, in accordance with a tilting movement of the self-standing inertia body, activates the lock mechanism to lock the extraction of the seat belt, wherein a shape of an outer perimeter of the supporting surface and a shape of an outer perimeter of the bottom surface are different from each other, and wherein the outer perimeter of one of the supporting surface and the bottom surface is configured such that a distance from a center axis of the self-standing inertia body to the outer perimeter of said one of the supporting surface and the bottom surface differs, in a horizontal plane, in a first direction and in a second direction orthogonal to the first direction.