PURPOSE: To make a form light in weight as well as to facilitate handling by making up a concrete form to be used for placing concrete by combining materials made of foamed styrol each of which is formed into specified shapes, with one another.

CONSTITUTION: A concrete form 10 for forming underground beams is made by combining a number of materials 12 each of which is made of foamed styrol and is formed into specified shapes with specified dimension, with one another. And both ends of each material 12 are formed with engaging sections 13 composed of a projected and a recessed section. In addition, separators 15 are provided, each of which keeps a space between the respective materials 12 at specified ones while combining them together, so that the parts of the respective engaging sections 13 are thereby combined with one another.