Pretensioner, seatbelt retractor including the pretensioner, and seatbelt apparatus including the seatbelt retractor

To increase the working amount by effectively transmitting energies of a plurality of force transmission members (12a) to a ring gear (15) for rotating a spool. A pipe (10) of a pretensioner (8) includes a cutout portion (10c) through which levers (17) of a ring gear (15) enter the pipe (10). An inner peripheral surface (10d) of the pipe (10) opposing the cutout portion (10c) includes a curved inner peripheral surface in a curved portion (10b) of the pipe (10), a linear inner peripheral surface in a linear portion (10a) of the pipe (10), and a transition portion P from the curved inner peripheral surface to the linear inner peripheral surface. The transition portion P is located closer to a position where balls (12a) start to press the levers (17) of the ring gear (15), than an intersection Q where an imaginary straight line ± extending from a cutout end portion (10h) of the cutout portion (10c) intersects an imaginary extension line ² of the inner peripheral surface (10d) of the linear portion (10a) of the pipe (10) at a right angle.