PROBLEM TO BE SOLVED: To facilitate assembling accuracy and manufacturing accuracy of a structure for raising a side floor part forward. SOLUTION: A back-raising drive mechanism for driving ups and downs a back floor portion of a floor plate, and an operating shaft 56 rotatably supported on the back surface of the central back portion of the back floor so that an axis extends along the width direction of the central back portion. , An arm 58 provided in the middle of the operating shaft, and one end fixed to the arm and the other end actuating a back-raising drive mechanism to drive the back floor part in a rising direction to rotate the operating shaft in a predetermined direction. When the wire 59 to be moved and the operating shaft are provided on both ends of the operating shaft so as to be able to change the angle with respect to the circumferential direction of the operating shaft, the back bed is driven in the upright direction by the angle and the operating shaft is rotated by the wire. , A pressing member positioned at either an operating position for rotating the pair of side back parts to the front or a non-operating position for not rotating the pair of side backs to the front. [Selection diagram]【課題】側部床部を前方へ起上させるための構造の、組み立て精度や製作精度の容易化を図るようにしたこと。【解決手段】床板体の背床部を起伏駆動する背上げ駆動機構と、背床部の中央背部の背面に軸線を中央背部の幅方向に沿わせて回転可能に支持された作動軸５６と、作動軸の中途部に設けられたアーム５８と、アームに一端が固定され他端が背上げ駆動機構を作動させて背床部を起上方向に駆動したときに作動軸を所定方向に回動させるワイヤ５９と、作動軸の両端部に作動軸の周方向に対して角度変更可能に設けられその角度によって背床部が起立方向に駆動されて作動軸がワイヤによって回動させられたとき、一対の側部背部を前方へ回動させる作動位置と、前方へ回動させることのない非作動位置とのいずれかに位置決めされる押圧部材を具備する。【選択図】 図６
