PURPOSE: To extend a life cycle of a pontoon by a method wherein standardization of materials and shortening of a manufacturing period of the pontoon are achieved by adopting a foam molded product as an inner form.

CONSTITUTION: A form of a pontoon is manufactured on a bottom plate with an inner form 2 and an outer form 3 which are composed of foam molded product. Reinforcement, steelwork, precast concrete, and prestressed sheath are assembled in a space between an outer face 4 of the inner form 2 and the outer form 3, and conrete is poured thereinto. After curing the concrete, the outer form 3 is demolished and carried out, and the pontoon is manufactured.