Abstract: A locking mechanism for looking a winding shaft wound a webbing to prevent the winding shaft from rotating at excessive movement more than a predetermined amount of acceleration in a horizontal direction. The locking mechanism has an operating member which is inclined at the excessive movement more than the predetermined amount of acceleration in the horizontal direction to lock the winding shaft. The operating body has a pair of plate-like inertia portions extended symmetrically with respect to the axis of the winding shaft in the direction to approach an outer periphery of the webbing wound around the winding shaft. The operating body is inclined by the inertia portions when the acceleration is exerted. Thus, the locking mechanism locks the winding shaft by means of these inertia portions and the locking mechanism can be made small in the vertical direction.