PURPOSE: To accurately position a core and also to smoothly enable extraction by connecting the prescribed working mechanism at a hooking part of the core providing a little looseness.

CONSTITUTION: In the hooking part 6a of the core 6, a working and 11a of cylinder 11 is connected through a core connecting body 14, connecting frame 12 and slider 13. And, a hooking piece 14b engaging with the connecting body 14, is arranged under fitting loosely to a hooking hole 14a, to enable a little looseness. In this constitution, after fitting the core 6 by stretching the working rod 11a, the core 6 is inserted into a cavity by drawing the rod in, to smoothly fit the hooking part 6a in a hooking hole 7a of a mold 2. Therefore, the core is smoothly and surely positioned and also at the time of extraction, the core is smoothly extracted by a little looseness.