A seat belt retractor has a ring gear connected for rotation with a seat belt reel. A planet gear and sun gear are freely rotatable about their axes. The planet gear is connected to a locking pawl, and a spring normally maintains the pawl in a release position. When the seat belt reel rotates at a rate of acceleration above a predetermined level, the inertia of the sun gear and the planet gear overcomes the action of the spring, causing the planet gear to move bodily around the sun gear. Movement of the planet gear moves the pawl to a lock position, in which it prevents withdrawal of the seat belt from the reel.