PURPOSE: To prevent the reduction of a thickness at a deep drawing part or a center bent part without increasing the thickness of a parison, by setting a core material having a skin presser provided at the desired position thereof to a blow mold and extruding a hollow parison from a molding machine to cover the core material and subsequently closing the blow mold and blowing compressed air in the blow mold to expand the parison.

CONSTITUTION: All of heat-extrudable and moldable thermoplastic resins can be used as a skin material but, for example, a polyvinyl chloride resin and a polypropylene resin etc. are pref. A skin presser 4 is constituted of a foam material having heat resistance and heat insulating property and the shape thereof is set so as to e adapted or made similar to that of a deep drawing part or a corner part and pref. circular or oval in general but not limited to this shape. The foam material is directly engaged with the core material but the core material and the foam material may be indirectly connected through a material such as an iron plate or a paper piece. As the foam material, for example, a polyurethane chip, polyethylene foam and polypropylene foam, etc. can be designated.