PURPOSE: To prevent the deformation of a core material in the crushing of the foam integrally molded along with the core material.

CONSTITUTION: Slide cores 9 are provided to an upper mold 8 and a cylinder mechanism 10 is connected to the slide cores 9. Communication passages 13 and valves 14 allowing a cavity C to communicate with the outside are provided to a lower mold 7. Insert wires 11 and frames 12 are set to the interior of the cavity C and a foamable resin is injected in the cavity C to close the lower and upper molds 7, 8. After a foam W3 is molded, the slide cores 9 are allowed to protrude into the cavity C by the cylinder mechanism 10 not only to allow the closed cells of the foam W3 to communicate with each other but also to compress the gas in the foam W3. The valves 14 are instantaneously opened to discharge the compressed gas in the foam W3 to the outside from the communication passages 13. By moving the compressed air in the foam W3 toward the communication passages 13, closed cells are allowed to communicate with each other over the whole of the foam W3. By properly arranging the slide cores 9 and the communication passages 13, the deformation of the insert wires 11 and the frames 12 can be prevented.