embodiment is equipped with a pull up spring 1b having an operating end 11b which actuates the belt in response to freeing of a preload set in advance a trigger 2b for maintaining the load in the set state by restraining the operating end 11b and for releasing the load from the set state by freeing the operating end from restraint and a sensor 3b for releasing the trigger 2b the trigger 2b is constituted by a toggle link mechanism having one end freely rotatably connected to the operating end 11b the toggle link mechanism comprises two arms 22b 25b the arm 22b has a base end connected to the operating end 11b of the pull up spring 1b as described above and the arm 25b has a base end connected to the operating end 11b of the pull up spring 1b as described above and the arm 25b has a base end freely rotatably supported on a pin protruding from a base not shown the arm 22b has a semicircular cylindrical surface 23b formed on its distal end and planar abutment surface 24 which is contiguous to the cylindrical surface 23b formed on the side opposite that on which the sensor means is disposed the arm 25b has a concave cylindrical surface 26b in sliding contact with the cylindrical surface 23b formed on its distal end and a.