To reduce the number of the points of components and to ease manufacturing precision by fixing a supporting leg between the top plate and the bottom plate of a housing part in the state of being freely rotatable and providing a housing part with a restraining member which is fit slidably in vertical direction but to the limit not to be out of joint and restrains the supporting leg at a storing position.

This system consists of a spindle part 10 fixing the supporting leg between the top plate 6 and the bottom 7 of the housing part to pivotally support it in the state of being freely rotatable, a supporting frame 12 formed to be freely movable by being fixed with a caster at its lower tip part, and a horizontal rod 13 connecting the part 10 and the frame 12. The upper tip part of the spindle part 10 is fit loosely to a recessed part 14 obtained by recessing the rear surface of the plate 6 to be vertically movably, the lower tip part is penetrated to an insertion hole 15 provided at the plate 7 and the hole 15 is covered by fixing a covering board to the rear surface of the bottom plate 7 to be pivotally supported between the plates 6 and 7. When the frame 12 is at the storing position, the restraining member 18 is in the shape of projecting a rib 18a at an L-shaped tip upper surface to be operated so as to carry the rod 13 upward.