PURPOSE: To reduce necessary numbers of supporting members and decrease necessary labor and costs, by using back up members composed of post members, bridging members, and jointing members, which can be stretched to be a ladder when used and folded when not used.

CONSTITUTION: Two post members 1 arranged in parallel with each other are jointed by a plurality of bridging members 2 to which jointing members 3 provided with a hinge-forming part 6 are fitted at both ends to constitute a back up member A which can be stretched to be a ladder when used and folded to be a parallelogram when not used. And the back up members A are arranged one by one in the respective middles of a plurality of molds 30 jointed in order to define the placing space of wall. And a plurality of square steel pipe 32 are straddled between mutual backs to press the mold plate 30 by use of separators, form ties 33, wedges 35, etc. In this way, necessary numbers of support members can be reduced and labor of construction, transportation, etc., can be reduced.