PURPOSE: To eliminate the manufacturing process of a core material through patchwork to improve the working efficiency, and remove the restrictions of a core material shape in terms of transportation or the like to manufacture an arbitrarily shaped pontoon by not pouring concrete in arranging bars around the core material, but pouring synthetic resin pre-foamed particles being a constitutional element of the core material into the Inner part of reinforced concrete.

CONSTITUTION: There is formed a reinforced concrete box 21 having a space 21d for housing a core material therein, and also having an opening 21e for making the space 21d communicate to the outer part, and then synthetic resin pre-foamed particles are put into the space 21d from the outer part 21e via the opening 21e and, following this, the opening 21e of the box 21 is blocked. The pre-foamed particles are press-fed into the space of the box under a pressure of the compressed air. The pre-foamed particles in the space are heated and then subjected to a secondary foaming.