PURPOSE: To prevent leakage of foam into a base material by using a slide means mounted on a cover of a mold, centering the material within a pocket, and forming a foam space for forming a foam layer between a skin layer cover and the material.

CONSTITUTION: After a plastisol skin layer cover 16 having a die fixing pocket 16b is formed, the cover 16 is charged on a support surface 18 of a mold cavity 12, and coincided. Then, a base material 20 is matched to the cover 16, and a hollow protrusion 20a of the material 20 is inserted into the pocket 16b of the cover 16. Thereafter, the cover 14 is closed, a centering tool 32 provided at the cover is operated, engaged with the protrusion 20a, and centered within the pocket 16b. a foam space 34 is formed between an inner surface of the pocket 16b of the sails 16a of the cover 16 and an outer surface of the protrusion 20a, and to prevent the foam from leaking from the space 34, a part of the surface of the material 20 is sealed with the flange part 15d of the sails 16a.