The conventional holding mechanism for foldable side rails have such problems that the release lever can be accidentally pressed for release action and that if a double lock is employed for preventing it, automatic holding action cannot be performed.This invention proposes a holding mechanism for foldable side rails, characterized in that the holding mechanism 4 is provided at an end of an upper horizontal rail member 1u and comprises a mechanism support member 5 connected with the stanchion member provided at said end; the mechanism support member 5 has a stopper pin set 12 capable of advancing and receding; the stopper pin set 12 is biased in the advancing direction by a spring 13; the stanchion member has a fitting hole 10 allowing the stopper pin set 12 to be fitted in it when the stanchion member is kept erect; the mechanism support member further has a release lever 6 engaged with a mechanically driven trigger site 15 provided in connection with the stopper pin set for making the stopper pin set recede against the force of the spring; a play mechanism is provided between the release lever and the mechanically driven trigger site; and the mechanism support member further has a release prevention mechanism for preventing the movement of the release lever in the release allowing direction.