To provide a method for thermoforming a resin sheet by which the surface of the resin sheet can be certainly protected and in turn, a successfully good-looking formed product can be manufactured without the remaining mark of a vent hole in the resin sheet, when the resin sheet is molded using a hotplate type vacuum/pressure forming process.

In this method, a protecting film comprising a sticky layer of the resin sheet side and a contact bonding layer formed of a poly(4-methylpentene-1) polymer resin composition of the hotplate side, laminated through an adhesive layer, is laminated on the resin sheet, when the resin sheet is molded using the hotplate-type vacuum/pressure forming process. In addition, the resin sheet is thermally softened by contact bonding the contact bonding layer of the protecting film to a hotplate and is shaped with the help of a mold.