Pretensioner, seat belt retractor having the same, and seat belt apparatus provided therewith

A pretensioner comprising:

• a pipe;
• a plurality of force transmission components that are provided so as to be movable in the pipe and transmit a force for rotating a spool in a direction in which a seat belt is taken up;
• a gas generator for generating gas in an event of an emergency;
• and
• a piston that is provided so as to be movable in the pipe and configured to, when activated by gas pressure resulting from the gas, exert a pressing force on the force transmission components,
• wherein the piston has an expanding section formed at an end thereof, the expanding section being configured to make contact with the force transmission components,
• wherein the expanding section has a recessed portion formed therein, and
• wherein the expanding section is configured such that a part of an inner circumferential surface of the recessed portion abuts on one of the force transmission components when the piston is operated by the gas pressure of the gas, and the expanding section is caused to expand toward an inner circumferential surface of the pipe by a section of said one of the force transmission components abutting on the inner circumferential surface of the recessed portion when the piston exerts the pressing force on the force transmission components and the one of the force transmission components advances into the recessed portion.