PURPOSE: To obtain a foaming unit of required strength by charging a foaming work site, whose foaming volume is regulated by a form or the like, with foaming resin material beads suitable for this work site, injecting heated gas to these material beads, and stirring and foaming them.

CONSTITUTION: Material beads 2 are input by a required amount into a charging work site 1 whose required volume is regulated, and it is covered with a mold plate 3 having steam vents 3a. Four injection nozzles 4 are arranged with an equal space in the mold plate 3 and provided toward a bottom part 1a of the charging work site 1. The injection nozzle 4 is connected to a pressure regulating valve 7, pump 6 and a boiler 5. Next, the material beads 2 are stirred in the charging work site 1, heated and foamed uniformly by injecting pressurized steam from the injection nozzle 4. The inside of the charging work site 1 is charged with a foaming unit in a condition that compression force is received by restricting the foaming volume. In this way, the charged foaming unit can be formed in a ground constitutional member having required strength.