A plurality of driving force transmitting members, a piston, a spring and a gas generator are inserted into the pipe from the end of the pipe where the gas generator is mounted. The gas generator is press-fitted in the pipe while the flat faces thereof are deformed by a pair of protrusions of the pipe. Accordingly, the gas generator is temporarily held by the protrusions of the pipe. Then, one end of the pipe is firmly pressed and the gas generator is mounted in the pipe.