PURPOSE: To manufacture a molded piece having an undercut part by a comparatively simple structure wherein a knockout pin is projected at an undercut molded part to separate the molded piece.

CONSTITUTION: After synthetic resin is injected into a cavity and molded, a movable die 3 is driven and opened and a block die 4 is lifted by a driving means 17 through an upper and a lower plate 15, 16. The undercut molding part 7 of the block die 4 projects slightly from the cavity of the fixed die 1. Then, a knockout pin 9 is projected by driving an ejector plate 20 from a guide hole 8 and a molded piece 6 is separted by elastic deformation from the under- cut molding part 7. In this way the molded piece having the under-cut part is manufactured simply without using a slide core of complicated structure or the like.