To simplify structure and to reduce cost by preventing rotation even when a round gas spring is utilized.

This device is provided with a supporting member (such as a structure) 2 composed of at least double cylinders fitted so as to be expanded/ contracted and a gas spring 15 incorporated in the supporting member 2 so as to expand/contract the supporting member 2. In that case, the cross-sectional contour of an inside member 22 of the supporting member 2 is composed of a non-circular member, a slide bearing face 24c in the same shape as the cross- sectional contour of the inside member 22 is provided, and at least one slide bearing 24 is provided while being fixed on an outside member 21 of the supporting member 2 for supporting the inside member 22 so as to slide it. Then, the inside member 22 slid relatively to the outside member so as not to be rotated in contact with the bearing face 24c on the inner peripheral surface of this slide bearing 24.