a cylinder - piston type seat belt tensioning device comprises a broach pipe wherein a broach rod which is connected to a seat belt is disposed , a load limiting member connecting the broach rod to the piston , which is broken at a predetermined load , and locking means for locking the broach pipe in the cylinder when the load limiting member is broken . after the load limiting member is broken at a predetermined load , the broach rod returns while plastically deforming the broach pipe to absorb the kinetic energy of the occupant .