In the case of a severe collision, the severity of the collision exceeds a first threshold value at which a pretensioner mechanism starts to activate, and then exceeds a second threshold value within a collision-discriminating time range T following the start of the activation; therefore, the force limiter mechanism unit is set at a high load. In the case of a gentle collision, the severity of the collision exceeds the first threshold value, at which the pretensioner mechanism starts to activate, but does not exceed the second threshold value within the collision-discriminating time range T following the start of the activation; therefore the force limiter mechanism unit is set at a low load. Therefore, without a determination delay, the switching of the force limiter mechanism can be appropriately selected.