Seatbelt retractor

A seatbelt retractor comprising:

• a windup drum that winds webbing;
• and
• a pretensioner unit that causes said windup drum to rotate in a windup direction of said webbing, wherein
• said pretensioner unit includes:
• a pinion gear configured to rotate in association with said windup drum;
• a piston that includes a rack for meshing with said pinion gear and causes said windup drum to rotate in said windup direction through said pinion gear;
• a tubular cylinder that movably holds said piston;
• a gas generating member that fills an interior space of said cylinder with a gas generated to drive said piston by a pressure of said gas, and
• a block member arranged adjacent to a part of the side surface of said cylinder,
• said tubular cylinder includes, on a side surface thereof, a through hole for releasing said gas to an outside of said cylinder,
• said through hole is provided to be located:
• in an initial position of said piston before gas generation, at a position at which the interior space of said cylinder and said outside are not continuous with each other through said through hole;
• and
• after gas generation, at a position at which the interior space of said cylinder that has changed upon moving of said piston and said outside enter a continuous state to be continuous with each together through said through hole, and
• in a circumferential direction of the side surface of said cylinder, said through hole is provided at such a position that said gas is released as an exhaust gas to a specific space defined by an inner wall of said block member and said part of the side surface of said cylinder.