To prevent an overhang door from being brought into contact with a door storage case panel or an intermediate stay, and to reduce the thickness of a partition by preventing the overhang door from being vibrated.

A positioning member 14 to position a lower end of the intermediate stay 3 and a guide member 18 having a guide roller 19 fitted to a guide groove 20 at a lower end of the overhang door 8 to guide the overhang door 8 on its upper surface are provided on a floor surface between a pair of forward and rear intermediate stays 3.