Various means for detecting the load on the bottom of a bed are proposed, but they have various problems, since the load sensor is provided separately from a bed or installed outside a bed.This invention solves the abovementioned problems by a load sensor-containing actuator in which one end side of a screw shaft 1 is rotatably supported by a base body 2; a female threaded member 3 is engaged with the screw shaft; and an actuation rod body 4 fixed to the female threaded member can advance and retreat in the axial direction of the screw shaft, wherein the base body is provided with a thrust bearing 6 coaxially with the screw shaft and an actuation cylinder 7 supporting the one end side of the screw shaft in the axial direction via the thrust bearing; a detection cylinder 9 is provided between the actuation cylinder and a support member 8 for the base body; and a strain gauge 10 is attached to the outside surface of the detection cylinder.The present invention relates to a foldable side rail for a bed or the like, particularly a foldable side rail in which multiple strut members are pivotally rotatably connected between an upper crosspiece member and a lower crosspiece member to form a parallel link mechanism.