PURPOSE: To make it possible to carry out online coating by means of blow molding, by constituting a core layer with a polyamide resin wherein an inorganic filling material is added and a skin layer with a polyamide resin, a polypropylene resin or an acrylic resin containing no filling material.

CONSTITUTION: As a resin A used for a core layer, polyamide resins such as polyamide 6, polyamide 66, polyamide 11, polyamide 12,aromatic polyamides and so on are used. As a filling material added in the resin A, calcium carbonate, talc, glass fibers, silica and so on are used. As a resin B used for a skin layer, beside the polyamides used as the resin A, polypropylene resins and acrylic resins are used. If the thickness of the skin layer is too thin, the appearance of the skin layer is influenced by the roughness of the filling material in the core layer and therefore the thickness is preferably 0.2mm or more. If the thickness is too thick, the rigidity is lowered and therefore it is preferably 1.0mm or less. Further, when 5W40% inorganic filling material are added to the resin A, the coating appearance is remarkably improved due to formation of the skin layer when multi-layer blow molding is done.