To realize reuse with less amt. of energy consumption and without requiring a large amt. of expenditure on installations by a method wherein a foamed plastic is compressed and steam at the m.p. of the plastic is fed when the temp. reaches the softening point and this plastic is furthermore compressed to perform compression molding.

A foamed polystyrene 1 ground or under used condition as it is fed into a compression molding machine S for compression molding of the foamed plastic to perform compression. Then, when internal temp. of the foamed polystyrene 1 is elevated by compression and reaches the softening temp., steam 5 at the same temp. as the m.p. of the foamed polystyrene 1 generated from a steam generating device is fed into the compression molding machine S. It is possible thereby to uniformly transmit heat to the gap parts of the foamed polystyrene by feeding the steam 5 at the same temp. as the m.p. of the foamed polystyrene 1 and after the steam is fed, it is furthermore compressed by means of the compression molding machine S to mold a rectangular parallelepiped.