PURPOSE: To eliminate the need for a separator or decrease the number of separators remarkably, and to shorten workhours by giving rigidity to forms by a rigid bar penetrated through a blank composed of foamed polystyrol in the vertical direction.

CONSTITUTION: Metallic groove members 12 consisting of two rows of iron, etc., are fixed onto a foundation section 10, a large number of blanks 13 made up of foamed polystyrol are inserted onto the groove members 12, and projecting sections 15 are engaged successively with recessed sections 16 and stacked and built. Rigid bars 18 such as an iron pipe are inserted into a plurality of through-holes 17 of the blanks 13 up to the groove sections 12 from an upper section, and a section between the upper sections of the rigid bars 18 approximately faced oppositely through a space 21 for placing concrete is connected at a proper interval by a connecting member 20 composed of iron, etc., thus constructing forms for a concrete structure. Accordingly, the forms are given large rigidity, no separator 25 is used or the number separator used is decreased and the forms are lightened and cost is reduced, and operation can be facilitated.