To maximize the transport efficiency and reduce the load in a construction site, by providing a plurality of panels corresponding to respective erected faces of a square column and connecting mutual side edges of adjacent panels with hinges and make them freely collapsible.

Four keystone plates 2, 2 whose upper end is cut to form a specified shape, are provided. And end part metallic pieces 4,... with a U-shaped section are fitted at the peripheral edges of respective keystone plates 2. Mutual end part metallic pieces 4 of the breadthwise edges of adjacent keystone plates 2 are connected to be horizontally rotatable by hinges 3.... As a result, all keystone plates 2 are connected one after another by the hinges 3 and hence, the whole form 1 can be collapsed in a plane. Accordingly, the form 1 can be transported in the minimum volume folded flatly and assembled in a three- dimensional form by only unfolding it in the construction site.