A seatbelt retractor comprising: a housing; a take-up drum that is rotatably supported at the housing and configured to be urged in a webbing-take-up direction while a webbing is wound therearound; and a pretensioner mechanism configured to force the take-up drum to rotate in the webbing-take-up direction in case of vehicle collision and takes up the webbing in the take-up drum, wherein the pretensioner mechanism comprises: a gas generating member configured to generate gas; a cylinder that is long and coupled with the gas generating member; a pinion gear body that is capable of rotating integrally with the take-up drum; a piston that is housed in the cylinder in a movable manner, pressed and configured to be driven by pressure of the gas, equipped with a rack getting engaged with the pinion gear body at a side surface portion thereof, and forced to rotate the take-up drum in the webbing-take-up direction via the pinion gear body when pressed and driven; and a sealing member that is a plate-like member having elasticity and a substantially similar shape with a gas-pressure-receiving-side surface of the piston, and mounted to the gas-pressure-receiving-side surface of the piston so as to seal a space between the piston and an inner wall of the cylinder, characterized in that the piston includes: a mounting concave portion having a predetermined depth at the gas-pressure-receiving-side surface thereof; and a communicating hole that is configured to perforate through the piston along a movement direction of the piston from the mounting concave portion, the sealing member includes a mounting convex portion erected at a position that is configured to meet with the mounting concave portion, and the sealing member is mounted to the piston with the mounting convex portion being press-fitted to the mounting concave portion, and the sealing member includes a gas releasing hole that is configured to perforate through the mounting convex portion in a thickness direction thereof. a housing; a take-up drum that is rotatably supported at the housing and configured to be urged in a webbing-take-up direction while a webbing is wound therearound; and a pretensioner mechanism configured to force the take-up drum to rotate in the webbing-take-up direction in case of vehicle collision and takes up the webbing in the take-up drum, a gas generating member configured to generate gas; a cylinder that is long and coupled with the gas generating member; a pinion gear body that is capable of rotating integrally with the take-up drum; a piston that is housed in the cylinder in a movable manner, pressed and configured to be driven by pressure of the gas, equipped with a rack getting engaged with the pinion gear body at a side surface portion thereof, and forced to rotate the take-up drum in the webbing-take-up direction via the pinion gear body when pressed and driven; and a sealing member that is a plate-like member having elasticity and a substantially similar shape with a gas-pressure-receiving-side surface of the piston, and mounted to the gas-pressure-receiving-side surface of the piston so as to seal a space between the piston and an inner wall of the cylinder, a mounting concave portion having a predetermined depth at the gas-pressure-receiving-side surface thereof; and a communicating hole that is configured to perforate through the piston along a movement direction of the piston from the mounting concave portion, the sealing member includes a mounting convex portion erected at a position that is configured to meet with the mounting concave portion, and the sealing member is mounted to the piston with the mounting convex portion being press-fitted to the mounting concave portion, and the sealing member includes a gas releasing hole that is configured to perforate through the mounting convex portion in a thickness direction thereof.