To prevent easy water intrusion during use, ensure good appearance, and easily and economically manufacture by making a foaming body of a liquid foaming material contained in a hollow box body.

Regarding the manufacture of a pontoon, a hollow box body 20 of concrete is prepared. The hollow box body 22 is formed to a preset shape, and internally divided into rooms 22 using divisions 21. A filling hole 25 for a liquid foaming material and a vent hole 26 are formed on the upper surface 24 of the hollow box body 20. The rooms 22 of the hollow box body 20 is filled with a liquid foaming material P from a filling machine T set to the filling hole 25 and foamed. As a result, the rooms 22 of the hollow box body become full of the liquid foaming material P and a synthetic resin foaming material 30 adheres to the internal surface 23 of a room. As a result, water hardly enters when the pontoon is used and a buoyancy drop hardly takes place. Also, high buoyancy can be obtained over a long term, and high durability is obtained. In addition, the pontoon can be easily manufactured without any complicated process.