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; and a gas generating member that fills an interior space of said tubular cylinder with a gas generated to drive said piston by a pressure of said gas; said tubular cylinder includes, on a side surface thereof, a through hole for releasing said gas to an outside of said tubular 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 tubular 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 tubular 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, characterized in that the seatbelt retractor further comprises a block member arranged adjacent to a part of the side surface of said tubular cylinder, and in a circumferential direction of the side surface of said tubular 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 tubular cylinder. 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; and a gas generating member that fills an interior space of said tubular cylinder with a gas generated to drive said piston by a pressure of said gas; 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; and a gas generating member that fills an interior space of said tubular cylinder with a gas generated to drive said piston by a pressure of said gas; said tubular cylinder includes, on a side surface thereof, a through hole for releasing said gas to an outside of said tubular 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 tubular 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 tubular 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, characterized in that the seatbelt retractor further comprises a block member arranged adjacent to a part of the side surface of said tubular cylinder, and in an initial position of said piston before gas generation, at a position at which the interior space of said tubular 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 tubular 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, characterized in that the seatbelt retractor further comprises a block member arranged adjacent to a part of the side surface of said tubular cylinder, and in a circumferential direction of the side surface of said tubular 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 tubular cylinder.