To prevent the mirror surface properties from being damaged by a flaw or dust in injection molding simultaneous decoration accompanied by a vacuum forming process.

A molded decorative sheet S is formed by successively laminating a decorative layer 2 and a thermoplastic resin sheet 3 on the rear surface of a transparent acrylic resin sheet 1 being the uppermost surface layer and, after the surface of the transparent acrylic sheet is subjected to mirror surface processing, a temporary adhesive protective sheet 4 comprising a polyethylenic resin is thermally bonded to the transparent acrylic resin sheet 1 under pressure. In injection molding simultaneous decoration, the vacuum forming of the molded decorative sheet is performed in such a state that the temporary adhesive protective sheet is attached and, thereafter injection molding is performed after the temporary adhesive protective sheet is peeled to obtain a decorative molded product by injection molding.