A pretensioner comprising a gas generating unit; a pretensioner body to whose inside the gas is supplied; a spool driving member disposed inside the pretensioner body and causing a spool to rotate to retract webbing onto the spool; a gas flow rate adjusting portion disposed in the spool driving member and whose inner peripheral shape is set in accordance with a flow rate of the gas to be passed through the gas flow rate adjusting portion; and a filter unit having a vent portion that is communicated with a space between the gas generating unit and the spool driving member and is also communicated with the gas flow rate adjusting portion. The open shape of the gas flow rate adjusting portion has a portion that does not coincide in a plan view with the open shape of the vent portion, and the overall open area of the vent portion is set larger than the open area of the gas flow rate adjusting portion.