FORM MATERIAL

PROBLEM TO BE SOLVED: To realize an easy demolding and reuse without the breakage of a form material by a method wherein at least a portion contacting with a concrete structure is made of alkaline fissionable substance. SOLUTION: At least a portion contacting with a concrete structure, concretely a form material contacting with a placed yet unhardened concrete is made of alkaline fissionable substance. After assembling a form plate, at the predetermined position of which the form material 1 is arranged, a concrete is placed. Since the form material 1 is made of alkaline fissionable substance, the surface of the form material is hydrolyzed by the alkali component flowed out of the concrete until the hardening of the concrete. As a result, a gap 22 is formed between a throating part 21 and the surface of the form material 1. Accordingly, even when the concrete hardens, no bonding is prevent between the form material 1 and the throating part 21, resulting in realizing an easy demolding due to the formation of the gap at the contacting part with the concrete structure.