To provide a locking mechanism for the double-folding fixing of a seat for a vehicle.

The locking mechanism for the double-folding fixing of the seat for the vehicle comprises a stopper for lock-supporting the seat in a double folding state between a main bracket and a leg frame of a front end lock of the downside of the seat, and a strap for unlocking the stopper. After the double folding of the seat in the locking state of the front end lock, an insertion part of the stopper is locked so as to be inserted into a supporting bracket of the leg frame, and the seat double-folded in a state that the stopper is fixed by the supporting bracket is firmly supported. When the strap is pulled after slightly tilting the seat forward, the insertion part of the stopper is removed from the supporting bracket, the stopper rotates backward, and the double-folding fixing of the seat is released.