A pre-tensioner of a seat belt device capable of securely and rapidly transmitting torque between a driven shaft and a rotational ring when the pre-tensioner is activated. The driven shaft projecting from a reel for retracting a seat belt is inserted into a housing and the rotational ring is positioned to surround a gear of the driven shaft. The rotational ring is provided with five guide faces in the inner surface thereof and pins are inserted into pocket portions of the rotational ring. Each pin is held at the both ends by an annular portion of a body and a cover. Stepped portions are formed along peripheries of both ends of each guide face to form spaces.