PURPOSE: To prevent occurrence of relative positional change of a striker and a lock plate in a lock device even when a load from a car body rear part works on a seat back in a device to lock the seat back foldable forward at a standing state.

CONSTITUTION: This is a device to engage a striker 9 fixed on a car body side member with a lock plate 11 of a lock unit 10 at the time when a seat back is erected and to be locked in this engagement state, and the lock unit 10 is bolt fixed on a seat back frame through a base plate 17 furnished with a bending promotion part 52 vertical width of which is narrowed by a recessed part 51 on an extensible part 49, and simultaneously, it is constituted so that a bolt 50 fixing the extensible part 49 is fastened by way of putting it through a long hole 44 provided on the extensible part 49.