A known vehicle seat comprises a lower rail having a front end pivotally attached to a floor of the vehicle, an upper rail slidably engaged by the lower rail and fixedly secured to a seat cushion frame, and a coupling device provided on a rear end of the lower rail and configured to detachably engage a striker fixedly secured to the floor.  See JP2005-53246A, for instance.  This vehicle can be pivoted between a use position and a storage position around the front end of the lower rail once the striker is released from the coupling device. 
In such a seat, to minimize deformation of the rail device and the coupling device under the inertia force created at the time of a vehicle crash, the rail device and the coupling device are required to be built with a high level of stiffness.  Therefore, the weight and the manufacturing cost of the seat inevitably increase.