To simply and economically produce a member on the side of a compartment integrally having an air bag door part with constant quality.

When a member on the side of a compartment having an air bag door part formed thereto in a demarcated state by a break scheduled part broken when an air bag is expanded is produced by injection molding, a pointed projection 45 is preliminarily formed to the core mold 42 of an injection mold at the position corresponding to the break scheduled part. A two-layered skin 21 consisting of a surface layer 22 and a rear foam layer 23 is arranged in the cavity mold 43 of the injection mold so that the rear foam layer 23 is turned toward the core mold 42 and the tip of the projection of the core mold 42 is pressed to the rear surface of a skin by mold clamping to form a notch in the rear foam layer 23 of the skin and a molten synthetic resin P is subsequently injected into the injection mold and a core 31 having a slit 32 formed thereto by the projection 45 is formed on the rear side of the skin.