PURPOSE: To secure supporting rigidity for a main body frame and keep off any deformation at the time of conveyance as well as to make assembly at a factory performable by installing a door panel, capable of opening or closing, whose upper end side is supported on a header member and the lower end side on a sill member respectively, in the inner side of this main body frame, and installing each reinforced member in every corner of this main body frame.

CONSTITUTION: A main body frame 7 is made up of a symmetrical pair of side members 1, a sill member 3 whose both ends are supported on each lower part of these side members 1 and a header members 5 whose both ends are supported on the upper part. Reinforced members 11, 13 are installed in each corner part of this main body frame 7, securing its supporting rigidity and thereby getting rid of a fear of any deformation at the time of conveyance, and its assembly at a factory is made possible, thus installation work at a job site is sharply reduced. In addition, in the inner part of this main body frame, there is provided with a paired door panel 19, capable of opening or closing, those upper end side is supported on the header member 5 and the lower end side on the sill member 3 respectively. In consequence, this door 19 is hard to suffer from any damage at time of conveyance.