PURPOSE: To prevent defective adhesion and molding from generating, by a method wherein at the time of heating press molding of an inner trimming material into a desired form possessing a thick and thin parts, water content within a base of a corrugated cardboard is released reliably outside from within the inner trimming material and a heating press mold.

CONSTITUTION: A part of a bottom force 12 forming a thick part 1b of an inner trimming material 1 is formed into a movable mold 12b which is movable vertically. The thick part is heating-press-molded between a top force 13 and the movable mold 12b under a state where the movable mold 12b is moved upward. Steam and compressed air to be generated at the time of heating press molding of a base of corrugated cardboard of the thick part are released outside through an opening 21 formed between a stationary mold 12a of the bottom force 12 and the top force 13. Then the movable mold 12b is moved downward and the thin part 1a is heating-press-molded between the stationary mold 12a of the bottom force 12 and the top force 13.