A seat belt tensioning device having a piston return stop device including a tapered member with an inclined surface rising toward one end which is slidably disposed within an axial hole of a piston and is connected to the piston through a connecting member breakable by a predetermined tension force. The other end of the tapered member is connected with a seat belt. Stopper members are slidably disposed within radial guide grooves of the piston and at their outer ends are slidably in contact with an inner wall of a piston cylinder while their inner ends are in contact with the tapered member.When the tension force of the seat belt reaches a predetermined value at an urgent time, the connecting member is broken and the tapered member is moved within the piston in a direction reverse to the travelling direction of the piston to push the stopper members toward the cylinder wall. Then the outer ends of the stopper members cut into the cylinder wall so that the return of the piston is stopped with certainty.