To enhance dimensional accuracy in a method wherein a base material having a sound absorbing material layer is preliminarily arranged in a mold to form a foamed layer by foam molding.

A core material 1 is partially provided with core material projected parts 10 piercing the sound absorbing material layer 4 and foam molding is performed in such a state that the leading ends of the core material projected parts 10 are brought into contact with one molding surface 50. Since the core material receives foaming pressure in such a state that the core material projected parts 10 are brought into contact with one molding surface 50, the deformation of the core material 1 caused by foaming pressure is prevented.