Patent ID: 8371613

Claim:
A pretensioner comprising: a pipe; a plurality of force transmission members movably provided in the pipe and formed by balls that transmit a force for rotating a spool in a seatbelt retracting direction; a gas generator that generates gas in case of an emergency; a ring gear that is at least rotatable and has a plurality of internal teeth on an inner periphery and a plurality of pressed portions on an outer periphery; and a pinion that is provided in a spool-side member, has external teeth meshed with the internal teeth, and rotates the spool, wherein, in case of the emergency, the force transmission members press the pressed portions of the ring gear so as to rotate the spool in the seatbelt retracting direction, wherein the pipe has a cutout portion through which the pressed portions of the ring gear enter the pipe, wherein an inner peripheral surface of the pipe opposing the cutout portion includes a linear inner peripheral surface provided at a position where the force transmission members press the pressed portions of the ring gear, and linearly extending in a longitudinal direction of the pipe, a curved inner peripheral surface connected to the linear inner peripheral surface on an upstream side in a moving direction of the force transmission members, and a transition portion from the curved inner peripheral surface to the linear inner peripheral surface, and wherein the transition portion is located closer to a position where the force transmission members start to press the pressed portions of the ring gear than an intersection where an imaginary straight line extending from a cutout end portion of the cutout portion intersects an imaginary extension line of the linear inner peripheral surface at a right angle.