PURPOSE: To make unnecessary the grinding process of ingate mark of as-cast product regardless of the size of pipe joint, by providing ring groove or the like in the specified position in the core in the cavity forming a pipe joint to be composed with core, and providing the cavity of riser part and ingate on the upper surface of core as specified.

CONSTITUTION: A mold is horizontally fitted with a core 4 being centered on a parting plane 3 of top part 1 and bottom part 2, and has a cavity 5 to form a joint pipe blank having a processing face of female thread 11. An ingate 7 flowing into cavity 6 of riser part and end part of cavity 5 is provided on the upper surface of the core 4, and a ring groove 9 ranging from the end face 10 of cavity 5 to the processing face 8, or plural partially recessed grooves are provided in the outer circumference of the core 4. And the upper edge of said ingate 7 is connected to the end face 10 within the range of chamfering part 12 of female thread, while the lower edge is connected to said ring groove 9 or one of recessed grooves. Since the ingate 7 is provided in the range to be extiuguished by processing of the chamfering part 11 of the female thread 11, the grinding process for the mark left by the ingate 7 may not be made unnecessary.