PURPOSE: To obtain an internal thread production method for easily molding an internal threaded body and a mold therefor.

CONSTITUTION: In the molding of a molded internal threaded body 10 having an internal thread part wherein a helical part is formed on the inner wall surface thereof, a core 12 is inserted to a cavity formed by closing molds for molding the molded internal threaded body 10. The core 12 is formed by mounting a cylindrical part 16 on a core main body 14. On the outer peripheral side face of the cylindrical part 16, a helically projected and recessed part made of an elastic body which can be elastically deformed is formed for molding the helical part. Next, after a resin charged into the cavity is set, the fitted state of the helical part forming the internal threaded part with the helically projected and recessed part of the cylindrical part is released. The molds are opened while the cylindrical part 16 is so deformed that the core 12 can be extracted from the molded internal threaded body 10.