Pretensioner, retractor, and seat belt device

A pretensioner comprising:

• a ring gear to which a spool is rotatably disposed, the spool winding a webbing for restraining an occupant;
• and
• a power transmission unit for transmitting power to the ring gear in an emergency, wherein
• the power transmission unit includes a power transmission member that is a rod-shaped member and that is engageable with external teeth of the ring gear, a pipe that contains the power transmission member, a gas generator that is disposed at an end of the pipe, a piston that receives gas generated from the gas generator to press the power transmission member, a first stopper consisting of a tip part of the power transmission member, a second stopper that is a separate part from the power transmission member and the piston and is disposed therebetween the power transmission member and the piston, and a pretensioner cover that defines a passage for the power transmission member,
• wherein the second stopper is adopted to be bitten by the external teeth of the ring gear to stop rotation of the ring gear, after the first stopper has reached the end of the passage and has absorbed at least some of the kinetic energy of the power transmission member.