PURPOSE: To form a door-back keeping strength as a shelf and having an increased storing space by a method wherein a vacuum forming mold is provided with a retaining part capable of bridging a reinforcing part and a sheet between the reinforcing pieces is stuck closely to each other from projection.

CONSTITUTION: Synthetic resin sheet 13 is fixed at four corners, heated by a heater and platisized. Then, chamber 30 is lifted to be stuck closely to a clamping frame and the opening part of the chamber 30 is blocked by the sheet 13. The pressure in the chamber 30 is increased by the discharge of the air to blow the sheet 13. Forming mold 17 retaining reinforcing parts 16 is elevated and the plastisized and blown sheet 13 covers the vacuum forming mold 17. Since lower hollow portion 34 of the vacuum forming mold 17 is made vacuous, the sheet 13 is stuck closely to the forming mold 17, the sheet surfaces 14a, 14b located at the space from the surface of the forming mold 17 to the reinforcing part 16 are stuck to each other to produce a sticking shelf. Then, when the sheet 13 is cooled the mold 17 is lowered, and separated from the sheet 13, the door-back is formed as the reinforcing parts 16 are buried in the sheet 13.