A seat belt is secured to an anchor at one side of a seated person and extends across the seated person's body to a second anchor at an opposite side of the seated person to which the belt is also secured. A belt retention device is operable, when actuated in response to a sudden, rapid deceleration of the vehicle, to grip the belt between the anchors at a point of engagement adjacent to the one side of the seated person to cooperate with the second anchor in taking up any slack in a section of the belt between the point of engagement and the second anchor and also maintaining such section of the belt tightly engaged across the person's body.