A webbing retractor which can prevent gas, which is for rotating a take-up shaft in a take-up direction, from affecting other structural parts. In the webbing retractor, when a pretensioner mechanism operates, gas for operation within a cylinder is exhausted toward a pinion via a gas venting hole formed in a flange portion of a piston. Because a blocking wall is provided between the pinion and an acceleration sensor, exhausted gas passes between the pinion and the blocking wall, and is exhausted to an outer side of the webbing retractor from an upper end side of a cover plate or the like. In this way, blowing-out of the exhausted gas onto the acceleration sensor is blocked, and the exhausted gas can be prevented from affecting the acceleration sensor.