To provide a bag which is lightweight, strong, and safe, and especially suitable for housing a personal computer.

Sheet-like core material which is dispersingly distributed with tiny spherical cells independently at least in surface layer parts of foamed resin core material, is prepared through a process of mixing high density polyethylene and low density polyethylene, of adding a foaming agent for higher temperature and a foaming agent for lower temperature, and of heating to be expanded. The bag is characterized by having an exterior member formed with a laminated composite panel which is made up with the foamed resin core material and thin stainless steel plates bonded by thermal pressing onto the core material surfaces each through an adhesive resin film.