In the conventional holding mechanism for supporting strut members in a state just before the stowed state is reached, reliable operation is difficult.This invention proposes a foldable side rail comprising multiple strut members 2 pivotally rotatably connected between an upper crosspiece member 1u and a lower crosspiece member 1d, to form a parallel link mechanism, in such a manner that the side rail can be operated between a deployed state for use where the strut members are kept upright and a stowed state of disuse where the strut members are laid down, wherein a first holding mechanism 5 is provided for holding said strut members in an upright position and second holding mechanisms 6 are provided on the lower crosspiece member for supporting the strut members in a state just before the abovementioned stowed state is reached, each of the second holding mechanisms consisting of a holding piece pivotally rotatable between a holding position and a non-holding position, a spring for urging the holding piece toward the holding position, a locking mechanism for locking the holding piece in the non-holding position, and an unlocking mechanism.
