PURPOSE: To mass-produce simply watch cases and instrument cases different in designs by selecting already-provided pins adequately and arranging them detachably and adequately on a molding die.

CONSTITUTION: Pins 4 of various shapes are provided previously, selected freely and fitted adequately in slots 5 of the molding die 1. Then, the molding die 1 is preheated while a molded plate 10 is mounted on through holes 9A of a fixing plate 9 and heated. The molded plate 10 begins softening, air is supplied through the through holes 9A, the milddle part of the molded plate 10 is blown upward and expanded and at the same time a lifting plate 8 is lifted to stick the molding die 1 closely by the molded plate 10. Simultaneously with this close adhesion, the inner air is sucked through an air passage 7 to form prescribed mold 2 at the state of reduced pressure or vacuum.