To solve the problem that a sure operation is difficult in a conventional holding mechanism for supporting a support member in the vicinity before reaching a storage state.

In the foldable side rail, a plurality of support members 2 are turnably connected so as to configure a parallel link mechanism between an upper crossbar member 1u and a lower crossbar member 1d, an operation is made possible between a using state wherein the support members are raised and a storage state wherein they are brought down, and a first holding mechanism 5 for holding the support members in an raised attitude is provided. A second holding mechanism 6 for supporting the support members in the vicinity before reaching the storage state is provided on the upper side of the lower crossbar member, and the holding mechanism includes a holding piece configured so as to be turned between a holding attitude and a non-holding attitude, a spring for urging the holding piece in the direction of the holding attitude, a locking mechanism for locking the holding piece in the non-holding attitude, and an unlocking mechanism.