A seat belt device has a gas generator for generating a high pressure gas when the vehicle is in emergency, a cylinder for conducting the high pressure gas from the gas generator, a piston received movably within the cylinder and pressed and moved by the high pressure gas, and a drive unit for rotating a winding shaft of a seat belt retractor in a direction of removing the slack of a belt, using the movement of the piston, wherein the pretensioner is employed with the energy absorbing mechanism, for reversely rotating the winding shaft if a tension acting on the belt exceeds a predetermined value after removing the slack of the belt.