To repeatedly use a form by forming the form in a double structure having an outer form and an inner form detachably provided at an inner surface side of the outer form, thereby simply repairing it when the form is damaged or deformed.

The form 11 comprises a longitudinal flange molding form 12, a web molding form 13 and a lateral horizontal beam molding form 14. The form 11 has a double structure having an outer form 15 and an inner form 16. The form 16 is superposed on an inner surface side of the form 15 and clamped by fastening members 7 such as screws or the like. Insertion holes 15 of the members 17 are formed at the form 16. And threaded holes 19 are formed at positions corresponding to the holes 18 are formed at the from 15. A material of the form 16 is a steel iron, polymer material such as FRP or the like, light metal plate or the like such as aluminum, stainless steel or the like. As the member 17, a belleville spring in which a head is not projected to the inner surface side of the form 16 is suitably used.