A piston return stop device for seat belt tensioning equipment. Connected at one end to a seat belt through a cylinder is a piston having a tapered portion the diameter of which enlarges toward its other end. A cylindrical sliding member is slidably disposed between the cylinder and piston so as to slide together with the piston when the piston is thrust to tension the seat belt. A respective stopper member is slidably disposed within each guide groove radially perforated through the cylindrical sliding member and supported by the tapered portion of the piston.When the piston is thrust through the cylinder at an urgent time until the tension force of the seat belt reaches a predetermined value, the piston is stopped from moving within the cylinder furthermore and only the cylindrical sliding member slides within the cylinder due to its inertia force together with the stopper members. Then the stopper member is pushed up by the tapered portion of the piston to cut into the inner wall of the piston. As a result, the return of the piston is certainly and safely stopped.