PURPOSE: To improve the external appearance of a molded piece to be caused as a result of tool exchange by setting the tool of a molding device applied according to an opening is set or closed in the mold on the corresponding side of the inside of a cabinet.

CONSTITUTION: When an opening 3 is necessitated for the cabinet 1, a tool exchange part for the middle portion of a core mold 5 for the inside of the cabinet 1 is provided in the mold 7. Hereby, the cabinet 1 can be provided by injecting resin into the space between the cavity mold 6 and the tool of the core mold. When the opening 3 is not necessary for the cabinet 1, the mold 7 is changed to the tool exchange mold 7'. Thus, the cabinet 1' can be formed. A label 12 is attached to the cavity 11 which is the recess for tool exchange, thereby obtaining better external appearance of the position where the tool is exchanged.