A pretensioner for restraining an occupant with a seat belt by pulling the seat belt when an impact on a vehicle is generated. The pretensioner comprises a retaining ring having a ring to be in contact with an end of a seal member facing a protective member and a plurality of projections to be in press contact with the inner wall of a housing that project radially outwardly from the periphery of the ring, wherein the retaining ring is press fitted into the housing so that the seal member is fixed to the inner wall of the housing by the retaining ring.