PURPOSE: To make complete shielding possible against electromagnetic field by a method wherein two side walls are attached to a platform which is made up of transverse profiled rails, lateral profiled rails and a honeycomb plate that is held at its peripheral edges by the aforementioned profiled rails.

CONSTITUTION: Two side walls 10, 11 connected with each other by three platforms 12-14 formed of built-in sub-assemblies are provided. Each of the platforms 12-14 is built in the same structure which is constructed of two transverse rails, two lateral rails, four corner connectors and a honeycomb plate. For the bottom platform 12, the transverse rails 15, 16 and the lateral rails 18, 19 are combined with the corner connectors 17, and the honeycomb plate 20 is supported therewith. One side of the corner connector 17 is inserted into grooves of the lateral rails 18, 19 and the other side is inserted into grooves of the transverse rails 15, 16. Metal springs 21 are inserted into the groove of the lateral rail 18 to press the assembled platforms 12-14 against the side walls 10, 11.